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Elsevier Editorial System(tm) for Building
and Environment
Manuscript Draft
Manuscript Number: BAE-D-15-01421R1
Title: A review of high temperature cooling systems in tropical buildings
Article Type: Review Article
Keywords: High Temperature Cooling; Radiant cooling; air-water system;
DOAS; energy saving; Tropical Buildings
Corresponding Author: Mr. Esmail Mahmoudi Saber, Ph.D. Student
Corresponding Author's Institution: National University of Singapore
First Author: Esmail Mahmoudi Saber, Ph.D. Student
Order of Authors: Esmail Mahmoudi Saber, Ph.D. Student; Kwok Wai Tham,
PhD; Hansjürg Leibundgut, PhD
National University of Singapore
4 Architecture Drive
Singapore, 117566
25 November 2015
Dear Sir/Madam,
Editor of Building and Environment Journal,
Please find enclosed to this letter the revised version of the manuscript entitled as “A
review of high temperature cooling systems in tropical buildings” for exclusive consideration
of publication in the Building and Environment Journal. This research has been conducted in
the department of building in National University of Singapore. It is an extension of a research
project which was based in Singapore-ETH Centre, co-funded by the Singapore National
Research Foundation (NRF) and ETH Zurich in collaboration with department of building at
National University of Singapore.
In this review article, the recent studies on the applications of high temperature cooling
systems in the tropical context have been reviewed. The outcomes of this research brought
new insights into different aspects of implementing this cooling strategy in tropical buildings.
The objectives of this paper are aligned with focus of the journal regarding high performance
buildings and sustainable built environment. This article should be of interest to a broad
readership including those interested in high temperature cooling and radiant-convective
systems in buildings. Please find the details of report and results of the research in the
manuscript.
Thanks for considering this article. Please address all correspondence regarding this
manuscript to me through my email ([email protected]) or contact me on phone (+65-
92279136) if you need further information.
Regards,
Esmail M. Saber
Cover Letter
National University of Singapore
4 Architecture Drive
Singapore, 117566
25 November 2015
Dear Sir/Madam,
Editor of Building and Environment Journal,
We would like to appreciate the efforts of reviewers for examining the paper and providing
these constructive and helpful comments. Please find our response to the raised comments and
suggestions listed in the below table.
Best regards,
Esmail M. Saber
No. Editor’s Comments
Reviewer#2
1
Page 5: Line 7: supply temperature in
the range of 18 to 24 °C… At this
supply temperature range would there
be sufficient dehumidification to
avoid condensation
As implied in the question, the
dehumidification provided by air system
drops for higher supply air temperature.
The authors of the cited paper did not
mention any concern regarding to
condensation on radiant panel surface. This
could be due to the low latent load of the
lab or low dew point level at supply air.
Nevertheless, the PMV of indoor space was
not in comfortable range at supply air
temperature of 24 °C. These explanations
were added to the manuscript.
2
Page 18: line 1-5. For spaces…rather
than DOAS.. I would like to have
author's view on this claim.
This statement was made based on the fact
that reconditioning of indoor air is a
common practice in conventional ACMV
system in commercial buildings with
conventional façade airtightness. In high
temperature cooling design, sensible load
could be handled by radiant/convective
cooling while latent load needs to be still
handled by air system. If latent load of
space including infiltration of humid
outdoor air and human load could be
Detailed Response to Reviewers
handled by low volume supply air of
DOAS, there would be no need to
recondition indoor air. This explanation
was added to the manuscript.
3
Page 29: line 29.. best to be kept near
minimum.. This may not be true in all
the cases. This depends on several
parameters, including the internal
load, latent loads etc.
We agree with the comment. The statement
revised to make it clear that from energy
efficiency point of view, it is best to keep
the ventilation rate near the minimum
requirements of standards. In spaces with
high latent load, supply air volume to space
may be required to exceed this minimum
requirement. This explanation was added to
the manuscript.
Reviewer#3
1
The title of article
It is better to use the term "high
temperature radiant cooling systems"
because generally high temperature
cooling means the radiant cooling
system.
The term of “high temperature cooling
system” has been used in this paper to
include all the systems which use higher
chilled water temperature than conventional
system. Heat transfer in HTC systems like
radiant panel or slab cooling happens
mostly through radiation while active
chilled or passive chilled beam are mainly
convective based. If we replace this term
with “high temperature radiant cooling
system”, it would not be inclusive of
convective based high temperature cooling
systems like active chilled beam or passive
chilled beam which were investigated in
this review paper.
2
In chapter 2, you can include the
activity of IEA Annex 59 and ISO
standardization for radiant heating
and cooling system. You can include
some references such as ISO
standards.
The information regarding activities in IEA
ECBCS group on application of low exergy
cooling systems in building was added to
the manuscript. The ISO 11855 guideline
on design, dimensioning, installation and
control of radiant cooling system has also
been referenced.
3
It is better to include the difference
between temperate climate and
tropical climate by analyzing the
standard climate data.
A graph has been added to the manuscript
to illustrate the temperature and dew point
variation during the year in the tropics as
well as in the dry and temperate climates
(Fig. 1).
4
It is better to focus on the specific
considerations for the application of
high temperature radiant cooling
system in tropical buildings. For this
purpose, the core findings of this
review article should be revised
accordingly. It is better to change the
core findings by the specific
considerations in tropical buildings
More specific considerations regarding
implementation of high temperature cooling
in tropical buildings were added to the
manuscript. This information relates to the
tropical context on the basis of high dew
point level all the year round, avoiding
condensation risk, preference of locally
acclimatized occupants, low lift chiller
operational conditions, potential connection
according to the different climate
condition. For example, due to the
very hot and humid climate
condition, thermal output from the
radiant surface can be restricted
according to the comfort and
condensation problems.
to cooling tower and other related aspects to
HTC system operations in this climate.
5
In chapter 5, it is better to suggest the
schematic diagrams for the
application of high temperature
radiant cooling system in tropical
buildings, and compare the pros and
cons each other. And then you can
discuss the design and operation
guidelines.
More explanations on the pros and cons of
various components and design strategies in
high temperature cooling implementation in
the tropics were added to the manuscript.
These aspects include potential energy
saving, thermal comfort, air quality, initial
and maintenance costs of different design
components and strategies.
6
In conclusions, it is better to suggest
the key findings for the application of
high temperature radiant cooling
system in tropical buildings. Now it
is a little bit general outcome.
Most of the key findings of this review are
exclusive to the tropics, while some of them
are applicable to the temperate and dry
climates as well. The outcomes in the
conclusion are revised to indicate whether
that statement is applicable to the tropics
only.
Department of Building,
National University of Singapore
4 Architecture Drive
Singapore, 117566
23 September 2015
Dear Sir/Madam,
Editor of Building and Environment Journal,
The core findings of this review paper could be summarized as follows,
Studies on applications of high temperature cooling in the tropics were reviewed
High temperature cooling can cut the air supply fan energy use by half
A custom designed low lift chiller can further utilize the potential energy saving
A detailed design/operation strategy of coupled air-water systems is essential
Membrane based air to air heat exchanger showed promising results for this context
Regards,
Esmail M. Saber
Highlights (for review)
A review of high temperature cooling systems in tropical buildings
Esmail M. Saber*1,2
Kwok Wai Tham1 Hansjürg Leibundgut
3
1 Department of Building, School of Design and Environment, National University of Singapore, Singapore
2 Singapore-ETH Centre for Global Sustainability, Future Cities Laboratory, Swiss Federal Institute of Technology (ETH), Zurich,
Switzerland 3 Institute of Technology in Architecture, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
* Corresponding author. Tel.: +6592279136
E-mail address: [email protected] (Esmail M. Saber).
Abstract
High temperature cooling is gaining more attention in commercial buildings of the tropical
climates where temperature and humidity is high all year round. In this air-water system,
radiant-convective cooling is provided into conditioned space through using higher chilled
water temperature compared to conventional all air system. Radiant cooling panel, radiant slab
cooling, passive/active chilled beams are the main design strategies for implementing this
concept into buildings. This paper reviewed and summarized the recent published papers on
applications of high temperature cooling systems in tropical buildings. The reported outcomes
and conclusions from these studies were extracted and discussed to get a better understanding
on overall performance of the systems which are designed based on this concept. The potential
energy saving of this strategy was estimated to be in the range of 6 to 41 % depending on
design strategies and operational scenarios of system. Comfortable and healthy indoor
environment is achievable for this design when a parallel air system satisfies latent load and
ventilation requirement of space. Low air movement was the only reported comfort concern
for the tropicsthis design since locally acclimatized occupants in the tropics prefer higher air
movement compared to dry and temperate climates. Regarding the parallel air system strategy,
DOAS with ceiling supply-ceiling exhaust was is suggested to be the best choice for
couplingto be coupled with high temperature cooling system. In addition, incorporation of
energy recovery systems like membrane based air to air heat exchanger into DOAS can
improve the overall efficiency of this design.
Revised Manuscript with Changes MarkedClick here to view linked References
Keywords –High Temperature Cooling; Radiant cooling; air-water system; DOAS; energy
saving; Tropical Buildings
1. Abbreviations
ACB Active Chilled Beam ACMV Air Conditioning and Mechanical Ventilation AHU Air Handling Unit CAV Constant Air Volume CC Chilled Ceiling CFD Computational Fluid Dynamics CE Ceiling Exhaust COP Coefficient of Performance CHWS Chilled Water Supply CHWR Chilled Water Return CS Ceiling Supply DDOAS Decentralized Dedicated Outdoor Air System DOAS Dedicated Outdoor Air System DV Displacement Ventilation EA Exhaust Air ERS Energy Recovery System FS Floor Supply IAQ Indoor Air Quality HTC High Temperature Cooling HTCW High Temperature Chilled Water HVAC Heating, Ventilation, and Air Conditioning LTC Low Temperature Cooling LTCW Low Temperature Chilled Water MF Mechanical Fan MHX Membrane Heat exchanger NV Natural Ventilation OA Outdoor Air PCB Passive Chilled Beam PMV Predicted Mean Vote PPD Predicted Percentage Dissatisfied RA Return Air RAS Recirculated Air System RCS Radiant Cooling System RCP Radiant Cooling Panel RDD Rotary Desiccant Dehumidifier RSC Radiant Slab Cooling SA Supply Air TABS Thermally Activated Building System VAV Variable Air Volume
2. Introduction
Our collective concern regarding to the global warming is slowly changing the way we
behave and act in different aspects of our lives in order to reduce our daily CO2 footprints.
Governments started proactively encouraging communities and companies to implement green
technologies at different sectors including building industry. In the tropical context where
temperature and humidity are high all year round, air conditioning and mechanical ventilation
systems (ACMV) is a necessity for commercial buildings. Several governmental and
International reports in tropical countries revealed that ACMV system consumes around half
of electricity use in commercial buildings [1]. Central all air system is the conventional
cooling system in tropical buildings where central chilled water plant provides chilled water
for air handling units which covers several thermal zones. The amount and temperature of
conditioned air to each zone could be controlled by variable or constant air volume
(VAV/CAV) through a user controlled thermostat located inside the conditioned space. High
temperature cooling (HTC) concept introduces a new design by using higher temperature
chilled water (≈ 16 °C) compared to the conventional design (≈ 6 °C). HTC design
incorporates a water based system to provide sensible cooling in the conditioned space while a
parallel air based system usually handles latent load and ventilation requirements of indoor
space. The energy performance superiority of this design comes from the facts that water is a
more efficient medium for heat transfer compared to air and high temperature chilled water
can be provided at higher chiller COP.
The implications of HTC strategies like radiant cooling panel, radiant slab cooling,
passive/active chilled beam have been extensively explored for the temperate and dry
climates. Novoselac and Srebric [2] provided a dimensionless performance metric of chilled
ceiling combined with displacement ventilation and concluded that it may or may not save
energy compared to all air system depending on operational parameters like supply air
temperature and outdoor flow rate. Tian and Love [3] conducted energy simulation analysis
for application of radiant slab cooling and showed that energy saving potentials range between
10 to 40 % for different climate types. In another study, Mumma [4] estimated that VAV costs
about 29 % more to operate compared to DOAS-RCP system for humid subtropical climate of
Philadelphia in US. He also argued that it takes hours for condensation film to appear on
radiant panel in the case occupancy level increases by a factor of 2 or 3 from design level [5].
The operational conditions and arrangements of HTC and air system in the indoor space
play an important role on indoor air characteristic indices like thermal stratification and
ventilation effectiveness [6]. Chiang et al. [7] evaluated the performance of CC-DV for the
subtropical climate of Taiwan and suggested supply temperature in the range of 18 to 24 °C to
reduce thermal stratification in the space. No concern of condensation on radiant ceiling
surface was reported for this range of supply temperature which could be due to the low dew
point level at supply air or low latent load of the lab. Nevertheless, the PMV of indoor space
was not in comfortable range at supply air of 24 °C. The results of their CFD simulation also
showed that floor air supply in this design performs better than ceiling supply. On the
contrary, Wang and Tian [8] concluded that ceiling delivery and ceiling exhaust is the
optimized arrangement for a hybrid radiant cooling and DOAS system. Based on their
findings, impact significance of design factors has an order of air supply temperature > panel
coverage area > panel temperature > air supply volume. In addition, Schiavon et al. [9] found
a strong correlation between indoor thermal stratification and average temperature of radiant
panel surface for CC-DV design. The shift in cooling strategy from all air system to air-water
system has also some impacts on heat gain level of buildings. It has been reported that there is
an increase of building cooling load for the implemented case of radiant cooling system and
efforts should be made to avoid direct sun shines on HTC system [10]. Feng et al. [11]
developed a simulation tool to investigate the magnitude of this cooling load difference
between radiant cooling and all air system. They found 5 to 15 % increase in average diurnal
cooling load compared to all air system and the level of increase was predicted to be higher for
floor cooling system.
In this paper, the reported studies in the literature on the applications of high temperature
cooling in the tropical context were reviewed. The scope of this review was limited to the
research studies conducted in or for the tropical climate based on the Köppen-Geiger climate
classification [12]. The group A of this classification includes tropical rainforest, tropical
monsoon, and tropical wet and dry (savanna) climates which are characterized as high
temperatures (≥18 °C) and precipitations all year round except for dry season in monsoon and
savanna climates. These climates usually occur in the areas near the equator and cover various
countries in Asia, Africa and America like Singapore, Malaysia, Thailand, Indonesia,
Philippine, Brazil, and India. The average monthly variation of temperature and dew point for
three climates of tropical rainforest (Singapore, Kuala Lumpur, Jakarta and etc.), hot desert
(Phoenix, Doha, Dubai and etc.) and warm temperate (Hanoi, Hong Kong, Taipei and etc.) are
illustrated in Fig. 1. In the tropical climate of Singapore, temperature and dew point remain
almost unchanged in the course of a year while in two other cities there is a clear warm season
which happens between May to September. The warm temperate climate of Hanoi exhibits
similar profile to the tropics during warm season where dew point is close to 25 °C. On the
other hand, at hot desert climate of Phoenix in Arizona, dew point level is unlikely to exceed
15 °C.
Fig. 1 The monthly variation of temperature and dew point for three cities of Singapore
(average of 1956-2015), Phoenix (average of 1948-2015) and Hanoi (average of 1959-2015)
[13]
The google scholar website has been chosen as the main platform and searches were
conducted based on combined keywords of “high temperature cooling”, “radiant cooling”,
“slab cooling”, “chilled beam”, “tropics” and “hot and humid”. Thirty eight38 papers on high
temperature cooling system in the tropics and 45 papers on generic applications of HTC have
been selected in the early stages. Based on relevance and originality of investigations, 27 and
17 papers were chosen for final investigation respectively for tropical and generic applications
of HTC. The outcomes and conclusions from the reported studies were extracted and
discussed on different aspects of HTC applications including energy saving, thermal comfort,
design strategies and operational scenarios. A collection of design strategies and system
components was introduced for efficient implementation of high temperature cooling systems
in tropical buildings based on the collected information from the papers.
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2. High temperature cooling in tropical buildings
2.1 Energy Saving Potential
High temperature cooling systems have the potential to reduce power consumption of
ACMV systems in buildings by using high temperature chilled water as heat transfer medium.
In the tropical context, high humidity ambient air condition requires a more advanced design
and control strategy in order to avoid condensation risk. Different amounts of energy saving
was reported in the literature for the implementation of high temperature cooling systems in
the tropics. The percentages of energy saving for the main reported studies in the literature are
listed in Table 1. Majority of studies found the HTC systems more energy efficient compared
to the conventional system. The level of savings ranged between 6 to 41 % while in some
design or operational scenarios, this concept consumed even more energy. The conventional
systems in the tropics are usually central all air system for office use and split unit air
conditioner for residential applications.
Table 1 List of reported studies in the literature on high temperature cooling systems applications
in tropical buildings
Authors
(year)
HTC
system
Air system
/
distribution
City / Climate
Type
Investigation
approach
Supply
water to
HTC
Energy
saving
Ameen and
Khizir [14][13] RCP RDD/DV
Pulau Pinag, Malaysia / Tropical rainforest
Experimental
setup in a test
chamber
13 °C
(panel
surface
15-18 °C)
-51 %
to 35 %
Kosonen and
Tan [15][14] ACB
DOAS/CS-
CE
Singapore / Tropical rainforest
Field
measurement in
a case study 16-17 °C NA
Vangtook and
Chirarattanano
n
[16,17][15,16]
RCP MF
Central Thailand /
tropical savanna
Experimental
setup and
Building energy
modeling
24-25 °C -100 %
to 6 %
Wahed et al.
[18][17] ACB RDD
Singapore / Tropical rainforest
Building Energy
modeling 18 °C
10 to
20 %
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Binghooth and
Zainal [19][18] RCP RDD
Penang, Malaysia / Tropical rainforest
Experimental
setup in a test
chamber
6-10 °C
(panel
surface
14-18 °C)
NA
Saber et al.
[20][19]
Meggers et al.
[21][20]
Iyengar et al.
[22][21]
RCP DDOAS/FS
-CE
Singapore / Tropical rainforest
Experimental
setup in a test
chamber
17-19 °C
(panel
surface
18-22 °C)
NA
Yau and Hasbi
[23][22] RSC VAV
Kuala Lumpur, Malaysia / Tropical rainforest
Field
measurement in
two case studies 19 °C NA
Tantiwichien
and Taweekun
[24][23]
RCP MF
Songkhla, Thailand /
tropical monsoon
Field
measurement
and Building
energy
modeling
25 °C 41 %
Sastry and
Rumsey
[25][24]
RSC
and
PCB
DOAS Hyderabad-
India / tropical savanna
Field
measurement in
a case study 14 °C 34 %
Nutprasert and
Chaiwiwatwor
akul [26][25]
RCP MF-FCU
Bangkok, Thailand /
tropical savanna
Experimental
setup and
Building energy
modeling
21-25 °C NA
Seshadri et al.
[27][26] RCP VAV
Singapore / Tropical rainforest
Experimental
setup and
Building energy
modeling
15 °C 26 %
Khan et al.
[28][27] RSC DOAS-FCU
Hyderabad- India / tropical
savanna
Experimental
setup and
Building
Simulation tools
< 16 °C 17.5 to
30 %
Compared to conventional system, air-water systems can potentially save energy through
further use of water as heat transport medium in order to lower fan energy use and duct size.
The heat capacity of water (4.183 kJ/(kg.K) @ 20 °C) is four times more than air (1.005
kJ/(kg.K) @ 20 °C), and consequently the heat transfer exchange process is potentially more
efficient with water. The main opportunity for energy saving in high temperature cooling
design comes from downsizing the supply air fan. The radiant cooling system could satisfy
some part of space sensible load which brings the chance to reduce the amount of supply air
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volume or even just to limit it to ventilation air. In one study in the tropics, Vangtook and
Chirarattananon [17][16] conducted annual energy simulations based on measured data in a 16
m2 laboratory space in which radiant cooling panels were implemented as active wall and
ceiling panel. The results of simulations showed that total fan energy use can be cut to half by
using radiant panel instead of conventional air conditioning concept. In another study, Khan et
al. [28][27] investigated thermal performance and energy saving potential of radiant cooling
system in commercial buildings of Hyderabad in India through CFD and energy simulations.
Annual energy consumption data revealed that 70 % of fan energy consumption can be
reduced by implementing DOAS combined with slab cooling. While in both of reported
studies, water pump energy use in radiant cooling system increased almost by three times,
overall required energy for air circulation were much higher than water circulation. A research
group in the tropical climate of Singapore introduced the idea of decentralized DOAS
(DDOAS) combined with radiant cooling based on low exergy concept to bring chilled water
closer to the conditioned space and further reduce the fan energy use [29,30][28,29]. The
Energy Conservation in Buildings and Community Systems (ECBCS) group of International
Energy Agency (IEA) has provided several reports on implementation of low exergy cooling
systems in buildings [31–33]. High temperature radiant slab cooling has been categorized in
these reports as the low exergy cooling system where temperature lift is reduced through use
of higher chilled water temperature.
High temperature cooling concept can accommodate another aspect of energy saving in
refrigeration system of chiller compared to all air system. Incorporating radiant cooling
systems in buildings facilitates use of higher CHWS temperature which can potentially save
energy by using a custom designed chiller for low temperature lift conditions. This higher
evaporator temperature can reduce the compressor work because of lower operational
temperature and pressure lift. The refrigeration cycles on pressure-enthalpy diagrams for two
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cases of low temperature and high temperature cooling are shown in Fig. 12. As can be seen in
this graph, with reduction of lift between low and high pressure side of refrigeration cycle, the
compressor performs less work. This graph indicates the efficiency superiority of HTC
concept over conventional LTC air conditioning system.
Fig. 1 2 Impact of raising the evaporative temperature on refrigeration cycle diagram [34][30]
The high temperature cooling system is required to be complemented with a parallel air
system to provide a comfortable and healthy indoor environment. In the tropical context, the
air based system usually requires low temperature chilled water (LTCW) for an effective
dehumidification. So, the combinations of LTCW and HTCW systems are necessary to truly
exploit the potential energy saving of high temperature cooling concept. Saber et al. [20][19]
implemented two chillers for the combination of DOAS-RCS in the tropical climate of
Singapore. The schematic arrangement of these two chillers operating in parallel is shown in
Fig. 23. The low temperature lift chiller provided HTCW for radiant panel and the second high
lift chiller was designated for dehumidification of outdoor air in decentralized air supply units.
In another study, Khan et al. [28][27] also considered two parallel chillers to provide chilled
water for AHU and slab cooling systems, separately. The results showed that this design
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strategy can reduce the annual energy consumption of chillers by 20 % compared to the
conventional design.
Fig. 2 3 Schematic diagram of decentralized DOAS-RCS with two separate low lift and
high lift chillers [20][19]
2.2 Thermal comfort and indoor air quality
The main and only goal of any ACMV system is to provide comfortable and healthy
indoor environment for occupants. Radiant heat transfer contribution has been enhanced in
cooling of indoor space with high temperature cooling systems. This change results in a new
type of conditioned space with lower air movement and mean radiant temperature compared to
all air system. Thermal comfort and IAQ performance of this system could be evaluated based
on the International standards and local guidelines in tropical countries. Fanger’s PMV/PPD
model is the most commonly used thermal comfort model in different climate types. However,
the majority of comfort studies [35–38][31–34] in the tropics were concluded that locally
acclimatized occupants have different perception compared to prediction of PMV model. So, a
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better understanding on actual perception of occupants in the tropics may be obtained by
considering both the International and local standards’ criteria.
Various design scenarios and control strategies have been exploited for implementation
of HTC systems in tropical buildings. Thermal comfort was achieved adequately and
accurately in some design scenarios while in other cases, indoor air was comfortable only
under specific hours of day and occupant activities. In one strategy, chilled water supply
temperature to HTC is kept around 24 to 25 °C to be always above the outdoor dew point level
in the tropical context. In parallel, fresh and humid outdoor air is brought into space through
natural ventilation (NV) or mechanical fan (MF) for ventilation requirement or raising indoor
air movement level. While condensation was avoided with this tactic, the high chilled water
temperature put a restriction on capacity of HTC. This design strategy is suitable for spaces
with low heat gain and only can provide comfortable indoor air space for night time residential
applications under resting activity. From a favorable view, the required high temperature
supply water for HTC can be provided through passive chilled water systems like cooling
tower. Vangtook and Chirarattananon [16][15] investigated the application of radiant cooling
in the tropical climate of Thailand by using natural air for ventilation and 24-25 °C chilled
water for radiant panel. The results of experiments and simulation showed that the capacity of
system was inadequate during hot period of the day and acceptable PMV only achieved during
night time for a reclining person. In daytime occupancy and under office activity, personal
fans have been considered to achieve thermal comfort [17][16]. In a similar study, Nutprasert
and Chaiwiwatworakul [26][25] used supply chilled water temperature of 25 C in radiant
cooling panel combined with natural air for cooling of a space in Thailand. The results of
building performance simulations showed that with this strategy, people feel warm or too
warm, 71 % of the time in the year.
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The second and main stream design strategy in operating high temperature cooling system
in the tropics is to keep CHWS temperature to HTC below outdoor dew point. A parallel
conditioned air system is required to keep the indoor dew point level below HTC operational
temperature. It can be seen from the list of studies in Table 1 that the supply water temperature
for this strategy ranges between 14 to 21 °C and the parallel air system could be DOAS or
RAS. Almost all of the reported studies in the literature [15,21,22,25][14,20,21,24] concluded
that adequate comfort condition can be achieved in the tropics with this design scheme.
Kosonen and Tan [15][14] conducted field measurements for an implemented case of active
chilled beam in Singapore. They found that with off coil air temperature of 14 °C and supplied
chilled water of 17 °C, indoor air condition of 23 °C and 65 % relative humidity could be
achieved during daytime. In another field study, Sastry and Rumsey [25][24] conducted a side
by side comparison of radiant slab cooling and VAV all air system for the tropical climate of
Hyderabad in India. Radiant cooling design provided higher satisfaction among occupants
although ceiling fan was used in the indoor office space to raise the air movement level.
Low air movement in indoor space was the only identified comfort issue in the literature
for application of radiant cooling systems in the tropics. There is no minimum air velocity
requirement in the International standards like ASHRAE standard 55 [39][35] and ISO 7730
[40][36]. However, several local standards in tropical countries put some limit on the minimum
acceptable air velocity. Local Singapore standard of SS 554 [41][37] recommended indoor air
velocity to be in the range of 0.1 to 0.3 m/s. Malaysian local standard of MS 1525 [42][38] also
have the recommended range of 0.15 to 0.5 m/s for air movement level in the occupied space.
Yau and Hasbi [23][22] assessed thermal comfort and indoor air quality level in two green
buildings in Malaysia in which radiant slab cooling were implemented. The objective
measurements in the buildings revealed that indoor air condition provided by radiant slab
cooling is compatible with the criteria of local and International standards. The only identified
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deficiency for this concept was low air movement which was below acceptable range in local
standards. In another study, Kwong et al. [43][39] conducted a thermal comfort survey in a
green building in Malaysia with slab radiant cooling and found air velocity below the limit set
by local guidelines.
2.3 System design/operation strategy
As explained in section 2.2, there are two main strategies in operating high temperature
cooling systems in the tropics. In the first strategy, supply chilled water temperature to HTC is
kept above outdoor dew point level and in the second strategy chilled water temperature is
kept below outdoor dew point and dehumidification is required to avoid condensation. The
main advantages of first scheme are utilizing passive chilled water system like cooling tower
and less risk of condensation and system complexity. This design is mostly suitable for night
time applications of residential buildings and personal fan has been advised to be used in
parallel for day time applications in commercial buildings. The second strategy is able to
provide adequate and accurate comfort condition under any indoor/outdoor scenarios.
However, parallel operation of radiant cooling and air based dehumidification systems makes
the design and control of overall system more complicated.
Radiant cooling panel (RCP), radiant slab cooling (RSC), active chilled beam (ACB) and
passive chilled beam (PCB) are the common types of HTC strategies. Each system has its own
advantages and drawbacks which makes it more suitable for some applications. RCP is a
temperature controlled surface which is attached to a chilled water hydronic system and
provides cooling into space mainly through radiation rather than convection. The infrared
image of a radiant ceiling panel in a conditioned lab is shown in Fig. 34. It can be seen that
temperature varies over the surface and it is colder in the areas which is in contact with water
pipes. Based on ASHRAE guideline [44][40], by assuming indoor air and average interior
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surfaces temperature of 27 °C, capacity of radiant panel ranges between 15 to 115 W/m2 for
water supply temperature of 15 to 25 °C. For a combined radiant panel and natural air system
in the tropical climate of Thailand, radiant panel capacity was found to be in the range of 30-
40 W/m2 with water supply temperature of 24-25 °C [16,45][15,41]. In another study in the
tropical climate of Singapore, Saber et al. [20][19] concluded that from morning till afternoon
under various supply water temperatures, radiant ceiling panel heat flux varies between 9 to 43
W/m2.
Fig. 3 4 Infrared image of a radiant cooling panel hung on ceiling [20][19]
In radiant slab cooling (RSC), water pipes are embedded into structure of building and it
provides mostly radiant cooling from floor or ceiling. This system requires integration with
building structure and needs to be planned at design stage before concrete pour (Fig. 45). RSC
also employs thermal mass of structure in order to shift and reduce peak cooling load of
building. Compared to radiant cooling panel, this design is more suitable for spaces where
steady cooling is required for a length of time. RCP can adjust faster to change in cooling load
and have better acoustical dampening as well as the option to be integrated with other building
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systems like lighting. International standard organization on building environmental design
section provided guidelines for design, dimensioning, installation and control of embedded
radiant cooling system into structure of building as thermally activated building systems
(TABS) [46].
Fig. 4 5 Water pipes in slab cooling system [25][24]
High temperature cooling system can be implemented in buildings through convective
based system of chilled beam in which chilled water passes through a heat exchanger located
inside the conditioned space. Passive chilled beam is usually suspended on the ceiling and
provides sensible cooling through convective heat transfer with surrounding air. Warm air
adjacent to coils gets cooler and denser so it moves downward and causes a natural convection
flow in the indoor space. The perforated metal casing in Fig. 5 6 is for providing comfortable
cooling with aesthetic advantages and typically it has 50 % open area. The implication of
passive chilled beam has not been explicitly explored in the literature for the tropical context
while it is known that PCB can provide fast response cooling and is suitable for spaces with
high cooling load requirements [25][24].
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Fig. 5 6 A passive chilled beam in operation [47][42]
In high temperature cooling systems like RCP, RSC and PCB, water based and air based
systems are operating independently in parallel. In active chilled beam design, conditioned air
passes through heat exchanger to enhance heat transfer mechanism between air and pipes. The
schematic of an operating active chilled beam is shown in Fig. 67. The primary ventilation air
mixes with secondary room air to supply dehumidified and cool air into space. In the tropical
context, the main design consideration for implementation of ACB is to avoid condensation on
beam and maintain dry cooling in the units. Kosonen and Tan [15][14] investigated an
implemented case of ACB in Singapore office building and concluded that condensation in
ACB unit is possible to be avoided by minimizing infiltration, supplying sufficient
conditioned air and adequate control of system.
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Fig. 6 7 An active chilled beam in operation [48][43]
An air based system is required to work in parallel to high temperature cooling device to
handle latent load and ventilation requirement of indoor space. This air system could be a
central DOAS-VAV, RAS-VAV design or even a decentralized DOAS or FCU located near
the conditioned space. In the ideal operational scenario, air system design accommodates
utilizing the most achievable cooling capacity from high temperature cooling system and it
satisfies main part of space sensible load. In DOAS design, supply air is only limited to
ventilation outdoor air and it can bring the opportunity to downsize AHU and reduce duct size
and fan energy use compared to the all air system. Khan et al. [28][27] conducted energy
simulation analysis for applications of radiant cooling system in the tropics and showed that
radiant cooling combined with DOAS can provide 12.8 % higher energy saving compared to
RSC-FCU design. The coupled design of decentralized DOAS-RCP [22,49,50][21,44,45] can
bring chilled water closer to the indoor space by using small decentralized air supply units
integrated into building structure and further cut the fan energy use. DOAS design suits spaces
with air tight façades and low latent load where dehumidified fresh air could bring down
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indoor dew point below HTC operational temperature. For spaces with high infiltration rate,
reconditioning of return air (RA) in RAS design can result in a more efficient system rather
than DOAS. If latent load of space including infiltration of humid outdoor air and human load
could be handled by low volume supply air of DOAS, there would be no need to recondition
indoor air.
The air distribution type in high temperature cooling applications also varies depending on
types of HTC device and space cooling load characteristics. This could vary from mixing
strategy like ceiling supply – ceiling exhaust to displacement oriented distribution types like
DV or floor supply – ceiling exhaust. In the reported studies of HTC applications, air supply
temperature ranged between 14 to 19 °C and ventilation rate was around 1 to 2 lit/s/m2 or 5 to
10 lit/s/person based on 5 m2 per person occupancy rate. This level of ventilation is above the
minimum ventilation requirement of Singapore standard SS 553 [51][46] which is 5.5
lit/s/person. The chosen range of supply temperature in this concept may not satisfy the local
comfort criteria for distribution strategy of DV and FS-CE. A numerical study on the
implementation of DDOAS-RCS under FS-CE distribution [52][47] concluded that with off
coil air temperature of 14 °C, in the areas close to floor diffusers, air could be around 17 °C
which raises the concerns of cold feet for occupants seated there.
Dehumidification of ventilation air requires low temperature chilled water while HTC
system is designed to operate with high temperature chilled water. The selection of design and
control strategy for providing both LTCW and HTCW at the same time has an important
impact on optimal operation and energy efficiency of the system. The typical strategy is to
produce low temperature chilled water (6 °C) in central plants and employ three way control
valves to raise the supply temperature for HTC. The schematic of water hydraulic lines and air
ducts in a DOAS combined with radiant panel is shown in Fig. 78. The valve modulator (V2)
regulates the flow volume in mix of supply and return chilled water in pipes based on
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feedback from temperature sensors (T3 or T4). Sastry and Rumsey [25][24] explored the
efficiency and robustness of different control strategies and found that controlled valves based
on fixed return water temperature perform the best. The second strategy is to keep chilled
water loop of HTC and LTC separate and use a water to water heat exchanger to transfer the
cooling to HTC loop. The schematic of a VAV-RCP with two separate hydronic system loops
is shown in Fig. 89. The return water from AHU passes through a heat exchanger to provide a
supply chilled water of 15 °C for radiant ceiling panel operation. This strategy has been
implemented mostly in research labs to have a better control on operation of high temperature
cooling device [27,45][26,41]. The third strategy is to use two separate chillers and water
hydronic loops for HTC and LTC systems as explained in Section 2.1 (Fig. 23). This scheme
has the potential to introduce further energy saving opportunities especially for the
applications where more than one chiller has been designated for cooling of building.
Fig. 7 8 Water hydronic lines and air streams in a DOAS combined with radiant panel
[53][48] (SF: supply fan, RF: return fan, P 1-2: pumps, C/C: cooling coil, T 1-11: temperature sensors, D 1-2:
Dampers, V 1-2: valve modulators, EW: Enthalpy wheel, RP: radiant panel, CH 1-2: chillers)
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Fig. 8 9 Employing a heat exchanger to provide high temperature chilled water in a VAV-
RCP system [27][26]
Because of concerns regarding to condensation in tropical buildings, the start up and shut
down strategy of HTC system needs to be well planned and followed. Kosonen and Tan
[15][14] concluded that air based system needs to be switched on 30 minutes before water
based system in the morning in order to get rid of stored humidity in the space over the night.
This amount of time was estimated for a building with air tight façade and it could vary based
on characteristics of building. It is also advisable to switch off HTC device before air based
system to avoid any possible condensation after scheduled occupancy. In the case of slab
cooling system implementation, energy saving concerns also comes into play in choosing the
right start up and shut down strategy. Because of stored cooling capacity in slab due to thermal
mass, it is required to operate the radiant system in a shifted schedule compared to occupancy
schedule. One case study in the tropics recommended to switch on slab cooling system one to
two hours before occupancy and to switch it off one to two hours before unoccupancy [25][24].
Khan et al. [28][27] also explored various control strategies for operation of slab cooling
system in the tropical climate of Hyderabad in India and concluded that maximum energy
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saving can be achieved when slab cooling system shut off four hours before end of occupancy
schedule.
2.4 Investment and maintenance costs
Investment cost required for different HVAC systems is usually the key decision factor for
selection of a design especially when building investors and users are two different parties.
The cost of equipments and labors are geographic dependent and they also could vary based
on market demand and scale of manufacturing. A study conducted side by side comparison of
VAV and slab cooling system in the Hyderabad of India and concluded that capital cost of
DOAS-RSC is slightly lower than VAV system [25][24]. The cost saving opportunities from
reduction of AHU and duct size was about 6 % of total cost of ACMV system while radiant
piping, accessories and installation raised the investment cost for around 23 %. The study
considered 33 % reduction in HVAC low side work or commissioning cost in the case of
radiant cooling. In another study, Mumma [4] compared the first cost of DOAS-Radiant panel
with all air VAV system for a six storey building in the context of Philadelphia, in US. He
showed that total cost benefits of radiant cooling system through reduction of AHU, ductwork
and floor to floor height as well as integration with electrical and fire suppression services
could outweigh the added cost of radiant panels. However, for the application of active chilled
beams in the context of California in US, Stein and Taylor [54][49] estimated that total cost of
DOAS-ACB system is about 2.5 times of VAVR design. They considered additional labor and
subcontractors costs for ACB design and installation.
High temperature cooling concept is a new design for the tropical context and its practical
implications have not been investigated explicitly for this climate. Having cooling coil and
chilled water inside the conditioned space requires further inspection and possibly higher
maintenance cost. In the case of active and passive chilled beam, a regular cooling coil
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cleaning service is required to avoid losing part of units’ capacity over time because of
collected dust on coils. The panel metal sheet and chilled water pipe behind the radiant cooling
panel also may require regular cleaning service. In order to reduce the risk of condensation in
applications of slab cooling system in the tropics, it was advised to insulate all water pipe
connections and manifolds [25][24].
3. Integration with energy recovery systems
In applications of high temperature cooling system, DOAS as the parallel air based system
can provide better performance and lower first cost opportunities compared to RAS due to
reduction in the size of AHU, ducting and fan energy use. The enthalpy difference between the
two air streams of fresh outdoor and indoor exhaust is significant and an energy recovery
system can reduce part of ventilation cooling load. Different types of ERS devices have been
introduced and investigated in the literature and industry. Rotary desiccant dehumidifier
(RDD) is one of these designs which incorporates a rotary wheel with desiccant coated
honeycombs for exchange of humidity. In active type of rotary desiccants (Fig. 910), a heat
source is required to activate the regeneration air streams in order to enhance the mass transfer
mechanism in the wheel. The warm and humid process air gets dehumidified after exchange of
humidity with regeneration air and then it is supplied into indoor space. The required heat
source is the main constraints for this design which is not easily accessible in the tropical
climate. Alongside this design, there is passive type of rotary desiccant in which the wheel is
placed between OA and EA streams and energy recovery happens without help of heat source.
In the passive RDD applications, the wheel needs to rotate at much faster speed compared to
active types [55][50].
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Fig. 9 10 Active type of rotary desiccant wheel with air heater [55][50]
Several studies in the literature tried to explore the application of high temperature cooling
system combined with active rotary desiccant dehumidification in the tropics. Ameen and
Khizir [14][13] evaluated the performance of an active desiccant dehumidification system
coupled with chilled ceiling in the tropical climate of Malaysia. They found that this
combination has superior performance over conventional system in applications where high
ventilation rate is required like in theatres and hospitals. In another study in Singapore, Wahed
et al. [18][17] conducted energy simulation to investigate the performance of a thermally
regenerated desiccant system integrated with chilled beam in the warm and humid climates.
They modelled the active RDD with three different heating sources for regeneration process
including electric heater, solar collector and waste heat. The annual energy simulation and
economic analysis revealed that solar assisted dehumidifier integrated with active chilled
beam is the most cost effective design which consumes 10-20 % less energy than conventional
VAV system in the tropics.
Membrane based air to air heat exchanger is another type of energy recovery systems
which is getting wide range applications in the industry. In this design, heat and humidity
transfers between two air streams happen at a porous membrane surface without any active
parts inside the device. Air streams inside the heat exchanger can interact in different
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arrangements as parallel, counterflow or cross flow streams [56][51]. During this interaction,
outside air loses some level of its heat and humidity to dry and cool exhaust air leaving the
indoor space. Nasif et al. [57][52] evaluated the performance of a Z type parallel plate heat
exchanger (Fig. 1011) made of porous membrane. In this Z type design configuration, three
arrangement scenarios of parallel flow, counter flow and cross flow happen between two air
streams inside the heat exchanger. The results of experiments and annual energy simulations
showed that implementing this enthalpy heat exchanger in air conditioning system can result
in 5.7 to 9 % energy saving for the tropical climate of Kuala Lumpur.
Fig. 10 11 “Z” type membrane based heat exchanger [57][52]
4. Direct connection to cooling tower
Incorporating high temperature cooling systems in building could bring the opportunity to
implement passive chilled water systems like cooling tower to provide the required HTCW
without use of refrigeration system. Outdoor wet bulb temperature is the key parameter for
this design which usually ranges between 25 to 26 °C in the tropics except in the dry season
for the tropical savanna climate. Cooling tower can provide chilled water in the range of 25 to
26 °C for the radiant cooling system. However, this level of chilled water temperature cannot
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provide effective cooling in HTC device for day time applications in office spaces. Vangtook
and chirarartaran [17][16] investigated different operational and design scenarios for the
application of cooling tower to provide high temperature chilled water for radiant cooling
panel in Thailand. The results showed that this system can provide adequate indoor condition
for spaces with low heat gain in night time applications. Tantiwichien and Taweekun [24][23]
also investigated the performance of radiant cooling panel using chilled water from an
employed cooling tower in the tropical climate of Thailand through experiments and
simulations. The results showed that this design is able to provide comfortable indoor
condition during night time and early morning with 41 % energy saving compared to split-type
air conditioner. The performance of incorporated cooling tower in this design was around 60
% which varied depending on outdoor dry bulb and wet bulb temperatures.
5. Discussions
The whole idea behind implementing high temperature cooling systems in buildings is to
introduce a more energy efficient cooling system compared to the conventional all air system.
A precise and detailed design strategy needs to be followed in order to truly utilize the
potential benefits of HTC systems. It was shown by several studies in the literature
[17,28][16,27] that fan energy use can be reduced by 50 to 70 % through incorporating DOAS
combined with radiant cooling system. Downsizing fan and duct size in ACMV system
compared to conventional design is the key factor for achieving higher energy saving in this
concept. Providing an air tight façade for the indoor space is necessary to minimize the
infiltration rate and latent load of space, .especially in the tropical context where outdoor
humidity is high all year round. In the ideal operational scenario, the amount of supply air
volume is in the range of minimum ventilation requirement of space and HTC plays a
significant role in satisfying the sensible load of space.
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HTC systems requires operate with high temperature chilled water which can bring the
potential to enhance the operational efficiency of chiller in buildings. Two separate chillers
need could to be installed in buildings to provide LTCW and HTCW for air and water
systems, respectively. This design strategy suits the buildings where multiple chillers were
assigned to be operational. The potential energy saving of this separation can be understood
from the reverse theoretical thermodynamic cycle of Carnot. This cycle is the most efficient
possible set of processes to transfer a certain amount of heat from a cold source (QC-TC) to a
hot one (QH- TH) with a given amount of usable work (W). The COP of the cycle in the
tropical context (average outdoor temperature of 30 °C) for the low temperature (6 °C) and
high temperature (16 °C) cooling operations can be determined as follows,
Based on the reverse theoretical Carnot cycle, there is a 77 % (Eq. 2-3) increase in
operational COP of chillers for high temperature cooling applications in tropical buildings.
However, because of the exergy losses in different sub-processes inside the chiller, the COP of
actual refrigeration systems operating in cooling system of buildings are far below the
theoretical Carnot values. The level of increase in COP for high temperature condition
compared to low temperature cooling scenario is also less likely to be in the same order. The
chillers in building cooling system are usually designed and regulated to efficiently provide
chilled water at temperature of 6-7 °C which matches the requirement of conventional air
conditioning system in the buildings. Custom designed chillers for low temperature lift
conditions have been introduced and investigated by some research institutes and
manufacturing companies for high temperature cooling applications or other process cooling
requirements. Doyon [34][30] investigated the critical design parameters of centrifugal chillers
for superior efficiency at low temperature lift conditions. He found open-drive/gear-drive
compressors with variable orifice and oil education system as the most suitable features in
chillers for high temperature cooling applications. He also concluded that chillers with
hermetic drive motors would not be able to utilize the potential energy saving of HTC
operational conditions due to required high head pressure for cooling of motor windings. In
another investigation, Wyssen et al. [58][53] designed a chiller prototype for low temperature
lift conditions which was equipped with a semi-hermetic reciprocating compressor. The
outcomes of experiments showed that at evaporative temperature of 15 °C, the COP of this
chiller increases from 4 at 40 °C temperature lift to about 11.4 at 13 °C lift. The COP of this
custom designed chiller as well as ideal Carnot and typical chillers are plotted in Fig. 12. It can
be seen that performance curve of this prototype chiller fits in between typical chillers and
ideal efficiency curves. The low lift chiller performs better than air cooled/water cooled
chillers both in terms of COP value at any specific lift and increase rate for lower temperature
lift. This shows some part of exergy losses in chiller sub-processes has been eliminated in this
design. The COP of this custom designed chiller as well as ideal Carnot and a regular chiller
are plotted in Fig. 11. It can be seen that the performance curve of this prototype chiller fits in
between regular chiller and ideal efficiency curves. The low lift chiller performs better than
regular chiller both in terms of COP value at any specific lift and increase rate for lower
temperature lift. This shows some part of exergy losses in the chiller sub-processes has been
eliminated.
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Fig. 11 12 COP of ideal Carnot cycle, low lift chiller and two typical air cooled/water
cooled chillers a regular chiller at evaporative temperature of 15 °C
The application of high temperature cooling in tropical buildings can be categorized based
on their supply chilled water temperature to HTC device. When the supply water temperature
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is above the outdoor dew point level and in the range of 25 to 26 °C, it can only provide
comfortable indoor space for night time applications. This design scheme could also be used
during day time in office buildings with low space heat gain for occupants under light clothing
level when personalized or ceiling fans are used to raise the air movement on work stations. In
the second design scheme, chilled water is supplied to HTC at temperatures lower than
outdoor dew point and comfortable indoor space can be achievable under any indoor/outdoor
scenarios. In this scheme, the parallel conditioned air system would keep the indoor dew point
below HTC operating temperature to avoid condensation. The only reported deficiency in the
literature for this conditioned indoor space is the low air movement as locally acclimatized
occupants in the tropics prefer to be exposed to higher air speed. This issue has been resolved
by using a ceiling fan in the office space for an implemented case of radiant cooling system
[25][24].
Different strategies have been investigated and suggested in the literature for the design
and operation of HTC in tropical buildings. A detailed control strategy is required to be
followed especially in the cases where HTC operational temperature is below outdoor dew
point to avoid condensation. The start up and shut down schemes could vary based on type of
HTC system and characteristics of buildings. In the case of slab cooling system, a shift needs
to be considered between operational schedule of HTC and occupancy period. For other types
of HTC systems located in the indoor space, the air based system should be switched on /off
before/ and after water based system in the morning and evening, respectively. The choice of
air conditioning and distribution system could also play an important role in energy saving and
thermal comfort of occupants. DOAS was found to be the best parallel air based system where
and from energy efficiency point of view, the amount of supply air is best to be kept near
minimum ventilation requirements of space. In spaces with high latent load, supply air volume
to space may be required to exceed this minimum requirement. Since, in this design scenario,
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the off coil temperature would be in the range of 14 to 16 °C, it is advisable to implement
ceiling supply-ceiling exhaust distribution instead of displacement strategy to avoid cold feet
for occupants.
The initial cost of high temperature cooling system varies based on HTC device types and
radiant slab cooling is the least costly choice. However, this design needs to be planned at
early stages before construction of building structure. The added cost of piping and accessories
to overall ACMV design could be mitigated by reduction of fan and duct size for circulation of
air in buildings. Less floor to floor height and integration with lighting and fire suppression
systems are other potential opportunities for saving material and space per floor area. In order
to utilize these potential benefits for implementation of high temperature cooling system, the
coordination of building structural, mechanical, electrical and fire protection contractors is
required. The maintenance cost of HTC system is also expected to be higher due to
significantly higher water pipes compared to conventional all air system especially in the case
of passive/active chilled beams.
The advantages of water based system combined with DOAS can be further utilized if an
energy recovery system is incorporated into air handling unit. The high enthalpy difference
between outdoor and indoor air in the tropics makes implementation of ERS more
economically feasible. Several studies in the literature reported significant energy saving for
coupling of HTC and active rotary desiccant dehumidification in the tropics through
experimental setup or simulation. Solar assisted heating was found as the best strategy for the
required heating in the regeneration process, although this source is not reliable in the tropics
due to high annual median cloud cover. Membrane heat exchangers (MHX) have also been
explored in the literature in the tropical context and promising performance was observed for
this design [50].
A collection of high temperature cooling design strategies and components in tropical
building based on reported outcomes in the literature iswas selected as shown in Fig. 1213.
User controlled temperature and humidity sensors are considered in indoor space to provide
necessary changes on operation of water and air based systems based on different
indoor/outdoor scenarios. User controlled temperature and humidity sensors were considered
in the indoor space to have separate feedback control on both water and air based systems.
With increase of humidity or dew point temperature above the specified threshold, signal
would be sent to actuators on air supply units’ pumps and three way control valve to reduce
water supply temperature and/or increase water flow rate. This change would enhance
dehumidification capacity of air system to avoid any risk of condensation on radiant panel.
Three way control valve and pump on HTC hydronic system could modulate its capacity
based on feedback from temperature sensors to provide the required sensible cooling for
space. Installing CO2 sensors can also bring further value to this design for the thermal zones
where there is high variation of occupancy level like in meeting rooms. Based on these
sensors’ feedback, the volume of fresh air coming into space could be regulated through
modulating supply air fan speed. With increase of humidity or dew point temperature above
the set threshold, damper in air duct gets more open to bring more dehumidified fresh air into
conditioned space. The three way control valve on HTC hydronic system modulates its
capacity based on the feedback from temperature sensors to provide enough sensible cooling.
In this design strategy (Fig. 13), anAn energy recovery system was is also designated for
DOAS to recuperate some part of cooling from exhaust air stream. This ERS device could be
an active/passive desiccant wheel or a membrane based air to air heat exchanger which
transfers heat and humidity between two air streams. The high enthalpy difference between
indoor and outdoor condition in the tropical context would make ERS an effective system for
the applications of DOAS. In addition, A central chilled water plant was assumed for this
design scenario and three way control valves were designated to provide the adequate chilled
water temperature for air based and water based systems. Tthe required high temperature
chilled water for HTC can be provided through alternative strategies like cooling tower or a
separate low lift chiller. Direct connection of HTC to cooling tower without refrigeration
system is mostly suitable for spaces with low heat gain in night time applications. A custom
designed chiller which operates at considerably higher efficiency in low temperature lift
applications can also be a more efficient solution compared to a central low temperature
chilled water (LTCW) system. The level of improvement in COP of designated low lift chiller
needs to outweigh the added cost of having a separate chiller for HTC, in order for this
alternative solution to be economically competitive
Fig. 12 13 The A selected design strategies and components for the application of high
temperature cooling in the tropics
6. Conclusions
The reported studies in the literature on the implementation of high temperature cooling
systems in the tropics were reviewed to get a better understanding on overall performance of
this cooling strategy for the warm and humid climates. Various aspects of this implementation
were explored in terms of energy saving and thermal comfort for locally acclimatized
occupants as well as optimal design and operational scenarios. The outcomes of this
investigation can be summarized in the following points,
HTC systems can bring the opportunity to downsize fan and duct size for air
circulation and cut the fan energy use by half compared to all air system
In the tropical context, aAn air tight façade is preferable for this strategy to
minimize the infiltration rate or in other words the latent load of space
Incorporating a custom designed chiller for low temperature lift conditions can
further utilize the potential energy saving of this concept
When supply chilled water temperature to HTC is above the outdoor dew point
level, comfortable indoor condition can be achieved only under specific
indoor/outdoor conditions. This high temperature water can be provided with
direct connection to cooling tower without use of refrigeration system in the
tropics.
When supply chilled water temperature to HTC is below the outdoor dew point
level, comfortable condition is achievable under any indoor/outdoor conditions.
Although a parallel conditioned air system is required to avoid condensation on
HTC device. The low air movement in this indoor space was the only reported
comfort concern for locally acclimatized occupants in the tropics.
A control strategy needs to be followed to avoid condensation in start up and shut
down period of system in tropical buildings. In the case of slab cooling system, a
shift between operational schedule of HTC and occupancy period needs to be
considered.
DOAS with ceiling supply-ceiling exhaust was found as the most suitable and
efficient air conditioning and distribution strategy for HTC implementation.
Among HTC cooling systems, slab cooling can be implemented at lower initial
cost. However, it requires early-stage planning and integration into structure of
building.
Incorporation of energy recovery system into DOAS can reduce ventilation
cooling load of space and improve the overall performance of HTC design.
Membrane based air to air heat exchangers showed promising performance in the
tropical context.
The practical implications of high temperature cooling system are required to be further
explored through actual implementation of this concept into tropical buildings. This coupled
air/water system demands a more sophisticated design and control strategy compared to all air
system. Thoughtful decisions need to be made at design stage based on experience of past
projects and building performance simulation for a successful implementation of high
temperature cooling strategy in the tropics.
7. Acknowledgment
This work was established at the Singapore-ETH Centre for Global Environmental
Sustainability (SEC), co-funded by the Singapore National Research Foundation (NRF) and
ETH Zurich in collaboration with department of building of NUS.
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[1] NEA. Singapore’s Second National Communication: Under the United Nations Framework Convention
on Climate Change. National Environmental Agency; 2010.
[2] Novoselac A, Srebric J. A critical review on the performance and design of combined cooled ceiling and
displacement ventilation systems. Energy Build 2002;34:497–509. doi:10.1016/S0378-7788(01)00134-7.
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through simulation. Elev. Int. IBPSA Conf., 2009.
[4] Mumma SA. Chilled Ceilings in Parallel with Dedicated Outdoor Air Systems: Addressing the Concerns
of Condensation, Capacity, and Cost. ASHRAE Trans 2002;108.
[5] Mumma SA. Condensation issues with radiant cooling panels. IAQ Appl 2001 2001:16–8.
[6] Saber EM, Mast M, Tham KW, Leibundgut H. Ventilation effectiveness and contaminant distribution in
an occupied space conditioned with low exergy ventilation technologies in the tropics, Eindhoven,
Netherlands: 2015.
[7] Chiang W-H, Wang C-Y, Huang J-S. Evaluation of cooling ceiling and mechanical ventilation systems on
thermal comfort using CFD study in an office for subtropical region. Build Environ 2012;48:113–27.
doi:10.1016/j.buildenv.2011.09.002.
[8] Wang W, Tian Z. Indoor thermal comfort research on the hybrid system of radiant cooling and dedicated
outdoor air system. Front Energy 2013;7:155–60. doi:10.1007/s11708-013-0244-z.
[9] Schiavon S, Bauman F, Tully B, Rimmer J. Room air stratification in combined chilled ceiling and
displacement ventilation systems. HVACR Res 2012;18:147–59. doi:10.1080/10789669.2011.592105.
[10] Olesen BW. Radiant Floor Cooling Systems. ASHRAE J 2008;50:16–22.
[11] Feng J (Dove), Schiavon S, Bauman F. Cooling load differences between radiant and air systems. Energy
Build 2013;65:310–21. doi:10.1016/j.enbuild.2013.06.009.
[12] Peel MC, Finlayson BL, McMahon TA. Updated world map of the Köppen-Geiger climate classification.
Hydrol Earth Syst Sci 2007;11:1633–44. doi:10.5194/hess-11-1633-2007.
[13] Ameen A, Mahmud K. Desiccant Dehumidification with Hydronic Radiant Cooling System for Air-
Conditioning Applications in Humid Tropical Climates. ASHRAE Trans 2005;111:225–37.
[14] Kosonen R, Tan F. A feasibility study of a ventilated beam system in the hot and humid climate: a case-
study approach. Build Environ 2005;40:1164–73. doi:10.1016/j.buildenv.2004.11.006.
[15] Vangtook P, Chirarattananon S. An experimental investigation of application of radiant cooling in hot
humid climate. Energy Build 2006;38:273–85. doi:10.1016/j.enbuild.2005.06.022.
[16] Vangtook P, Chirarattananon S. Application of radiant cooling as a passive cooling option in hot humid
climate. Build Environ 2007;42:543–56. doi:10.1016/j.buildenv.2005.09.014.
[17] Wahed MA, Wong YW, Toh KC, Ho HK. Performance Analysis of Thermally Regenerated Desiccant
System Integrated With Chilled Beam for Warm Humid Climate. ASME 2010 Int. Mech. Eng. Congr.
Expo., 2010, p. 1375–82. doi:10.1115/IMECE2010-40263.
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[18] Binghooth AS, Zainal ZA. Performance of desiccant dehumidification with hydronic radiant cooling
system in hot humid climates. Energy Build 2012;51:1–5. doi:10.1016/j.enbuild.2012.01.031.
[19] Saber EM, Iyengar R, Mast M, Meggers F, Tham KW, Leibundgut H. Thermal comfort and IAQ analysis
of a decentralized DOAS system coupled with radiant cooling for the tropics. Build Environ
2014;82:361–70. doi:10.1016/j.buildenv.2014.09.001.
[20] Meggers F, Pantelic J, Baldini L, Saber EM, Kim MK. Evaluating and adapting low exergy systems with
decentralized ventilation for tropical climates. Energy Build 2013;67:559–67.
doi:10.1016/j.enbuild.2013.08.015.
[21] Iyengar RS, Saber E, Meggers F, Leibundgut H. The feasibility of performing high-temperature radiant
cooling in tropical buildings when coupled with a decentralized ventilation system. HVACR Res
2013;19:992–1000. doi:10.1080/10789669.2013.826065.
[22] Yau YH, Hasbi S. Field analysis of indoor air quality in high rise and low rise green offices with radiant
slab cooling systems in Malaysia. Indoor Built Environ 2013:1420326X13506130.
doi:10.1177/1420326X13506130.
[23] Tantiwichien A-U-W, Taweekun J. An Experimental and Simulated Study on Thermal Comfort. IACSIT
Int J Eng Technol 2013;5:177–80.
[24] Sastry G, Rumsey P. VAV vs. Radiant Side-by-Side Comparison. ASHRAE J 2014;56:16–24.
[25] Nutprasert N, Chaiwiwatworakul P. Radiant Cooling with Dehumidified Air Ventilation for Thermal
Comfort in Buildings in Tropical Climate. Energy Procedia 2014;52:250–9.
doi:10.1016/j.egypro.2014.07.076.
[26] Seshadri B, Sapar MBH, Jian Z, Neth M, Wu B, Ng D. Feasibility Study of Chilled Ceiling Technology in
Singapore through Simulation and Verification. Build. Simul. Optim. Conf., London: 2014.
[27] Khan Y, Khare VR, Mathur J, Bhandari M. Performance evaluation of radiant cooling system integrated
with air system under different operational strategies. Energy Build 2015;97:118–28.
doi:10.1016/j.enbuild.2015.03.030.
[28] Bruelisauer M, Chen K, Iyengar R, Leibundgut H, Li C, Li M, Mast M, Meggers F, Miller C, Dino R,
Saber EM, Schlueter A, Tham KW. BubbleZERO—Design, Construction and Operation of a
Transportable Research Laboratory for Low Exergy Building System Evaluation in the Tropics. Energies
2013;6:4551–71. doi:10.3390/en6094551.
[29] Saber E, Meggers F, Iyengar R. The potential of low exergy building systems in the tropics - Prototype
evaluation from the BubbleZERO in Singapore. Proc. Clima 2013 Energy Effic. Smart Healthy Build.,
Prague, Czech Republic: 2013.
[30] Doyon T. Chiller Design For Low-Lift Conditions. Air Cond Heat Refrig News 2008;234:13–5.
[31] Indraganti M, Ooka R, Rijal HB, Brager GS. Adaptive model of thermal comfort for offices in hot and
humid climates of India. Build Environ 2014;74:39–53. doi:10.1016/j.buildenv.2014.01.002.
[32] Willem HC, Tham KW. Associations between thermal perception and physiological indicators under
moderate thermal stress. 6th Int. Conf. Indoor Air Qual. Vent. Energy Conserv. Build. Sustain. Built
Environ., 2007.
[33] Gong N, Tham KW, Melikov AK, Wyon DP, Sekhar SC, Cheong KW. The Acceptable Air Velocity
Range for Local Air Movement in The Tropics. HVACR Res 2006;12:1065–76.
doi:10.1080/10789669.2006.10391451.
[34] Andreasi WA, Lamberts R, Cândido C. Thermal acceptability assessment in buildings located in hot and
humid regions in Brazil. Build Environ 2010;45:1225–32. doi:10.1016/j.buildenv.2009.11.005.
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Occupancy. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers;
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determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and
local thermal comfort criteria. Geneva, Switzerland: ISO 2005; 2005.
[37] SS 554. SS 554 : 2009, Code of practice for indoor air quality for air-conditioned buildings. Singapore:
SPRING Singapore; 2009.
[38] MS 1525. Code of Practice on Energy Efficiency and Use of Renewable Energy for Non-Residential
Buildings. Putrajaya, Malaysia.: Department of Standards Malaysia; 2007.
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for Residential Building in a Tropical Climate. Jounal Autom Control Eng 2014;2:67–70.
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J 2007;49.
[43] Alexander D, O’Rourke M. Design Considerations For Active Chilled Beams. ASHRAE J 2008:50–8.
[44] Kim MK, Leibundgut H. Evaluation of the humidity performance of a novel radiant cooling system
connected with an Airbox convector as a low exergy system adapted to hot and humid climates. Energy
Build 2014;84:224–32. doi:10.1016/j.enbuild.2014.08.005.
[45] Kim MK, Leibundgut H. A case study on feasible performance of a system combining an airbox
convector with a radiant panel for tropical climates. Build Environ 2014;82:687–92.
doi:10.1016/j.buildenv.2014.10.012.
[46] SS 553. SS 553 : 2009, Code of practice for air-conditioning and mechanical ventilation in buildings.
Singapore: SPRING Singapore; 2009.
[47] Saber E, Mast M, Tham KW, Leibundgut H. Numerical Modelling of an Indoor Space Conditioned with
Low Exergy Cooling Technologies in the Tropics. 13th Int. Conf. Indoor Air Qual. Clim., Hong Kong:
2014.
[48] Jeong J-W, Mumma SA, Bahnfleth WP. Energy conservation benefits of a dedicated outdoor air system
with parallel sensible cooling by ceiling radiant panels. ASHRAE Trans 2003;109:627–36.
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Proc Clima, Antalya, Turkey: 2010.
A review of high temperature cooling systems in tropical buildings
Esmail M. Saber*1,2
Kwok Wai Tham1 Hansjürg Leibundgut
3
1 Department of Building, School of Design and Environment, National University of Singapore, Singapore
2 Singapore-ETH Centre for Global Sustainability, Future Cities Laboratory, Swiss Federal Institute of Technology (ETH), Zurich,
Switzerland 3 Institute of Technology in Architecture, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
* Corresponding author. Tel.: +6592279136
E-mail address: [email protected] (Esmail M. Saber).
Abstract
High temperature cooling is gaining more attention in commercial buildings of the tropical
climates where temperature and humidity is high all year round. In this air-water system,
radiant-convective cooling is provided into conditioned space through using higher chilled
water temperature compared to conventional all air system. Radiant cooling panel, radiant slab
cooling, passive/active chilled beams are the main design strategies for implementing this
concept into buildings. This paper reviewed and summarized the recent published papers on
applications of high temperature cooling systems in tropical buildings. The reported outcomes
and conclusions from these studies were extracted and discussed to get a better understanding
on overall performance of the systems which are designed based on this concept. The potential
energy saving of this strategy was estimated to be in the range of 6 to 41 % depending on
design strategies and operational scenarios of system. Comfortable and healthy indoor
environment is achievable for this design when a parallel air system satisfies latent load and
ventilation requirement of space. Low air movement was the only reported comfort concern
for this design since locally acclimatized occupants in the tropics prefer higher air movement
compared to dry and temperate climates. Regarding the parallel air system strategy, DOAS
with ceiling supply-ceiling exhaust is suggested to be the best choice to be coupled with high
temperature cooling system. In addition, incorporation of energy recovery systems like
membrane based air to air heat exchanger into DOAS can improve the overall efficiency of
this design.
*Revised Manuscript with No Changes MarkedClick here to view linked References
Keywords –High Temperature Cooling; Radiant cooling; air-water system; DOAS; energy
saving; Tropical Buildings
1. Abbreviations
ACB Active Chilled Beam ACMV Air Conditioning and Mechanical Ventilation AHU Air Handling Unit CAV Constant Air Volume CC Chilled Ceiling CFD Computational Fluid Dynamics CE Ceiling Exhaust COP Coefficient of Performance CHWS Chilled Water Supply CHWR Chilled Water Return CS Ceiling Supply DDOAS Decentralized Dedicated Outdoor Air System DOAS Dedicated Outdoor Air System DV Displacement Ventilation EA Exhaust Air ERS Energy Recovery System FS Floor Supply IAQ Indoor Air Quality HTC High Temperature Cooling HTCW High Temperature Chilled Water HVAC Heating, Ventilation, and Air Conditioning LTC Low Temperature Cooling LTCW Low Temperature Chilled Water MF Mechanical Fan MHX Membrane Heat exchanger NV Natural Ventilation OA Outdoor Air PCB Passive Chilled Beam PMV Predicted Mean Vote PPD Predicted Percentage Dissatisfied RA Return Air RAS Recirculated Air System RCS Radiant Cooling System RCP Radiant Cooling Panel RDD Rotary Desiccant Dehumidifier RSC Radiant Slab Cooling SA Supply Air TABS Thermally Activated Building System VAV Variable Air Volume
2. Introduction
Our collective concern regarding the global warming is slowly changing the way we
behave and act in different aspects of our lives in order to reduce our daily CO2 footprints.
Governments started proactively encouraging communities and companies to implement green
technologies at different sectors including building industry. In the tropical context where
temperature and humidity are high all year round, air conditioning and mechanical ventilation
systems (ACMV) is a necessity for commercial buildings. Several governmental and
International reports in tropical countries revealed that ACMV system consumes around half
of electricity use in commercial buildings [1]. Central all air system is the conventional
cooling system in tropical buildings where central chilled water plant provides chilled water
for air handling units which covers several thermal zones. The amount and temperature of
conditioned air to each zone could be controlled by variable or constant air volume
(VAV/CAV) through a user controlled thermostat located inside the conditioned space. High
temperature cooling (HTC) concept introduces a new design by using higher temperature
chilled water (≈ 16 °C) compared to the conventional design (≈ 6 °C). HTC design
incorporates a water based system to provide sensible cooling in the conditioned space while a
parallel air based system usually handles latent load and ventilation requirements of indoor
space. The energy performance superiority of this design comes from the facts that water is a
more efficient medium for heat transfer compared to air and high temperature chilled water
can be provided at higher chiller COP.
The implications of HTC strategies like radiant cooling panel, radiant slab cooling,
passive/active chilled beam have been extensively explored for the temperate and dry
climates. Novoselac and Srebric [2] provided a dimensionless performance metric of chilled
ceiling combined with displacement ventilation and concluded that it may or may not save
energy compared to all air system depending on operational parameters like supply air
temperature and outdoor flow rate. Tian and Love [3] conducted energy simulation analysis
for application of radiant slab cooling and showed that energy saving potentials range between
10 to 40 % for different climate types. In another study, Mumma [4] estimated that VAV costs
about 29 % more to operate compared to DOAS-RCP system for humid subtropical climate of
Philadelphia in US. He also argued that it takes hours for condensation film to appear on
radiant panel in the case occupancy level increases by a factor of 2 or 3 from design level [5].
The operational conditions and arrangements of HTC and air system in the indoor space
play an important role on indoor air characteristic indices like thermal stratification and
ventilation effectiveness [6]. Chiang et al. [7] evaluated the performance of CC-DV for the
subtropical climate of Taiwan and suggested supply temperature in the range of 18 to 24 °C to
reduce thermal stratification in the space. No concern of condensation on radiant ceiling
surface was reported for this range of supply temperature which could be due to the low dew
point level at supply air or low latent load of the lab. Nevertheless, the PMV of indoor space
was not in comfortable range at supply air of 24 °C. The results of their CFD simulation also
showed that floor air supply in this design performs better than ceiling supply. On the
contrary, Wang and Tian [8] concluded that ceiling delivery and ceiling exhaust is the
optimized arrangement for a hybrid radiant cooling and DOAS system. Based on their
findings, impact significance of design factors has an order of air supply temperature > panel
coverage area > panel temperature > air supply volume. In addition, Schiavon et al. [9] found
a strong correlation between indoor thermal stratification and average temperature of radiant
panel surface for CC-DV design. The shift in cooling strategy from all air system to air-water
system has also some impacts on heat gain level of buildings. It has been reported that there is
an increase of building cooling load for the implemented case of radiant cooling system and
efforts should be made to avoid direct sun shines on HTC system [10]. Feng et al. [11]
developed a simulation tool to investigate the magnitude of this cooling load difference
between radiant cooling and all air system. They found 5 to 15 % increase in average diurnal
cooling load compared to all air system and the level of increase was predicted to be higher for
floor cooling system.
In this paper, the reported studies in the literature on the applications of high temperature
cooling in the tropical context were reviewed. The scope of this review was limited to the
research studies conducted in or for the tropical climate based on the Köppen-Geiger climate
classification [12]. The group A of this classification includes tropical rainforest, tropical
monsoon, and tropical wet and dry (savanna) climates which are characterized as high
temperatures (≥18 °C) and precipitations all year round except for dry season in monsoon and
savanna climates. These climates usually occur in the areas near the equator and cover various
countries in Asia, Africa and America like Singapore, Malaysia, Thailand, Indonesia,
Philippine, Brazil, and India. The average monthly variation of temperature and dew point for
three climates of tropical rainforest (Singapore, Kuala Lumpur, Jakarta and etc.), hot desert
(Phoenix, Doha, Dubai and etc.) and warm temperate (Hanoi, Hong Kong, Taipei and etc.) are
illustrated in Fig. 1. In the tropical climate of Singapore, temperature and dew point remain
almost unchanged in the course of a year while in two other cities there is a clear warm season
which happens between May to September. The warm temperate climate of Hanoi exhibits
similar profile to the tropics during warm season where dew point is close to 25 °C. On the
other hand, at hot desert climate of Phoenix in Arizona, dew point level is unlikely to exceed
15 °C.
Fig. 1 The monthly variation of temperature and dew point for three cities of Singapore
(average of 1956-2015), Phoenix (average of 1948-2015) and Hanoi (average of 1959-2015)
[13]
The google scholar website has been chosen as the main platform and searches were
conducted based on combined keywords of “high temperature cooling”, “radiant cooling”,
“slab cooling”, “chilled beam”, “tropics” and “hot and humid”. Thirty eight papers on high
temperature cooling system in the tropics and 45 papers on generic applications of HTC have
been selected in the early stages. Based on relevance and originality of investigations, 27 and
17 papers were chosen for final investigation respectively for tropical and generic applications
of HTC. The outcomes and conclusions from the reported studies were extracted and
discussed on different aspects of HTC applications including energy saving, thermal comfort,
design strategies and operational scenarios. A collection of design strategies and system
components was introduced for efficient implementation of high temperature cooling systems
in tropical buildings based on the collected information from the papers.
2. High temperature cooling in tropical buildings
2.1 Energy Saving Potential
High temperature cooling systems have the potential to reduce power consumption of
ACMV systems in buildings by using high temperature chilled water as heat transfer medium.
In the tropical context, high humidity ambient air condition requires a more advanced design
and control strategy in order to avoid condensation risk. Different amounts of energy saving
was reported in the literature for the implementation of high temperature cooling systems in
the tropics. The percentages of energy saving for the main reported studies in the literature are
listed in Table 1. Majority of studies found the HTC systems more energy efficient compared
to the conventional system. The level of savings ranged between 6 to 41 % while in some
design or operational scenarios, this concept consumed even more energy. The conventional
systems in the tropics are usually central all air system for office use and split unit air
conditioner for residential applications.
Table 1 List of reported studies in the literature on high temperature cooling systems applications
in tropical buildings
Authors
(year)
HTC
system
Air system
/
distribution
City / Climate
Type
Investigation
approach
Supply
water to
HTC
Energy
saving
Ameen and
Khizir [14] RCP RDD/DV
Pulau Pinag, Malaysia / Tropical rainforest
Experimental
setup in a test
chamber
13 °C
(panel
surface
15-18 °C)
-51 %
to 35 %
Kosonen and
Tan [15] ACB
DOAS/CS-
CE
Singapore / Tropical rainforest
Field
measurement in
a case study
16-17 °C NA
Vangtook and
Chirarattanano
n [16,17]
RCP MF
Central Thailand /
tropical savanna
Experimental
setup and
Building energy
modeling
24-25 °C -100 %
to 6 %
Wahed et al.
[18] ACB RDD
Singapore / Tropical rainforest
Building Energy
modeling 18 °C
10 to
20 %
Binghooth and
Zainal [19] RCP RDD
Penang, Malaysia / Tropical rainforest
Experimental
setup in a test
chamber
6-10 °C
(panel
surface
14-18 °C)
NA
Saber et al.
[20] Meggers
et al. [21]
Iyengar et al.
[22]
RCP DDOAS/FS
-CE
Singapore / Tropical rainforest
Experimental
setup in a test
chamber
17-19 °C
(panel
surface
18-22 °C)
NA
Yau and Hasbi
[23] RSC VAV
Kuala Lumpur, Malaysia / Tropical rainforest
Field
measurement in
two case studies
19 °C NA
Tantiwichien
and Taweekun
[24]
RCP MF
Songkhla, Thailand /
tropical monsoon
Field
measurement
and Building
energy
modeling
25 °C 41 %
Sastry and
Rumsey [25]
RSC
and
PCB
DOAS Hyderabad-
India / tropical savanna
Field
measurement in
a case study
14 °C 34 %
Nutprasert and
Chaiwiwatwor
akul [26]
RCP MF-FCU
Bangkok, Thailand /
tropical savanna
Experimental
setup and
Building energy
modeling
21-25 °C NA
Seshadri et al.
[27] RCP VAV
Singapore / Tropical rainforest
Experimental
setup and
Building energy
modeling
15 °C 26 %
Khan et al.
[28] RSC DOAS-FCU
Hyderabad- India / tropical
savanna
Experimental
setup and
Building
Simulation tools
< 16 °C 17.5 to
30 %
Compared to conventional system, air-water systems can potentially save energy through
further use of water as heat transport medium in order to lower fan energy use and duct size.
The heat capacity of water (4.183 kJ/(kg.K) @ 20 °C) is four times more than air (1.005
kJ/(kg.K) @ 20 °C), and consequently the heat transfer exchange process is potentially more
efficient with water. The main opportunity for energy saving in high temperature cooling
design comes from downsizing the supply air fan. The radiant cooling system could satisfy
some part of space sensible load which brings the chance to reduce the amount of supply air
volume or even just to limit it to ventilation air. In one study in the tropics, Vangtook and
Chirarattananon [17] conducted annual energy simulations based on measured data in a 16 m2
laboratory space in which radiant cooling panels were implemented as active wall and ceiling
panel. The results of simulations showed that total fan energy use can be cut to half by using
radiant panel instead of conventional air conditioning concept. In another study, Khan et al.
[28] investigated thermal performance and energy saving potential of radiant cooling system
in commercial buildings of Hyderabad in India through CFD and energy simulations. Annual
energy consumption data revealed that 70 % of fan energy consumption can be reduced by
implementing DOAS combined with slab cooling. While in both of reported studies, water
pump energy use in radiant cooling system increased almost by three times, overall required
energy for air circulation were much higher than water circulation. A research group in the
tropical climate of Singapore introduced the idea of decentralized DOAS (DDOAS) combined
with radiant cooling based on low exergy concept to bring chilled water closer to the
conditioned space and further reduce the fan energy use [29,30]. The Energy Conservation in
Buildings and Community Systems (ECBCS) group of International Energy Agency (IEA) has
provided several reports on implementation of low exergy cooling systems in buildings [31–
33]. High temperature radiant slab cooling has been categorized in these reports as the low
exergy cooling system where temperature lift is reduced through use of higher chilled water
temperature.
High temperature cooling concept can accommodate another aspect of energy saving in
refrigeration system of chiller compared to all air system. Incorporating radiant cooling
systems in buildings facilitates use of higher CHWS temperature which can potentially save
energy by using a custom designed chiller for low temperature lift conditions. This higher
evaporator temperature can reduce the compressor work because of lower operational
temperature and pressure lift. The refrigeration cycles on pressure-enthalpy diagrams for two
cases of low temperature and high temperature cooling are shown in Fig. 2. As can be seen in
this graph, with reduction of lift between low and high pressure side of refrigeration cycle, the
compressor performs less work. This graph indicates the efficiency superiority of HTC
concept over conventional LTC air conditioning system.
Fig. 2 Impact of raising the evaporative temperature on refrigeration cycle diagram [34]
The high temperature cooling system is required to be complemented with a parallel air
system to provide a comfortable and healthy indoor environment. In the tropical context, the
air based system usually requires low temperature chilled water (LTCW) for an effective
dehumidification. So, the combinations of LTCW and HTCW systems are necessary to truly
exploit the potential energy saving of high temperature cooling concept. Saber et al. [20]
implemented two chillers for the combination of DOAS-RCS in the tropical climate of
Singapore. The schematic arrangement of these two chillers operating in parallel is shown in
Fig. 3. The low temperature lift chiller provided HTCW for radiant panel and the second high
lift chiller was designated for dehumidification of outdoor air in decentralized air supply units.
In another study, Khan et al. [28] also considered two parallel chillers to provide chilled water
for AHU and slab cooling systems, separately. The results showed that this design strategy can
reduce the annual energy consumption of chillers by 20 % compared to the conventional
design.
Fig. 3 Schematic diagram of decentralized DOAS-RCS with two separate low lift and
high lift chillers [20]
2.2 Thermal comfort and indoor air quality
The main and only goal of any ACMV system is to provide comfortable and healthy
indoor environment for occupants. Radiant heat transfer contribution has been enhanced in
cooling of indoor space with high temperature cooling systems. This change results in a new
type of conditioned space with lower air movement and mean radiant temperature compared to
all air system. Thermal comfort and IAQ performance of this system could be evaluated based
on the International standards and local guidelines in tropical countries. Fanger’s PMV/PPD
model is the most commonly used thermal comfort model in different climate types. However,
the majority of comfort studies [35–38] in the tropics were concluded that locally acclimatized
occupants have different perception compared to prediction of PMV model. So, a better
understanding on actual perception of occupants in the tropics may be obtained by considering
both the International and local standards’ criteria.
Various design scenarios and control strategies have been exploited for implementation
of HTC systems in tropical buildings. Thermal comfort was achieved adequately and
accurately in some design scenarios while in other cases, indoor air was comfortable only
under specific hours of day and occupant activities. In one strategy, chilled water supply
temperature to HTC is kept around 24 to 25 °C to be always above the outdoor dew point level
in the tropical context. In parallel, fresh and humid outdoor air is brought into space through
natural ventilation (NV) or mechanical fan (MF) for ventilation requirement or raising indoor
air movement level. While condensation was avoided with this tactic, the high chilled water
temperature put a restriction on capacity of HTC. This design strategy is suitable for spaces
with low heat gain and only can provide comfortable indoor air space for night time residential
applications under resting activity. From a favorable view, the required high temperature
supply water for HTC can be provided through passive chilled water systems like cooling
tower. Vangtook and Chirarattananon [16] investigated the application of radiant cooling in
the tropical climate of Thailand by using natural air for ventilation and 24-25 °C chilled water
for radiant panel. The results of experiments and simulation showed that the capacity of
system was inadequate during hot period of the day and acceptable PMV only achieved during
night time for a reclining person. In daytime occupancy and under office activity, personal
fans have been considered to achieve thermal comfort [17]. In a similar study, Nutprasert and
Chaiwiwatworakul [26] used supply chilled water temperature of 25 C in radiant cooling panel
combined with natural air for cooling of a space in Thailand. The results of building
performance simulations showed that with this strategy, people feel warm or too warm, 71 %
of the time in the year.
The second and main stream design strategy in operating high temperature cooling system
in the tropics is to keep CHWS temperature to HTC below outdoor dew point. A parallel
conditioned air system is required to keep the indoor dew point level below HTC operational
temperature. It can be seen from the list of studies in Table 1 that the supply water temperature
for this strategy ranges between 14 to 21 °C and the parallel air system could be DOAS or
RAS. Almost all of the reported studies in the literature [15,21,22,25] concluded that adequate
comfort condition can be achieved in the tropics with this design scheme. Kosonen and Tan
[15] conducted field measurements for an implemented case of active chilled beam in
Singapore. They found that with off coil air temperature of 14 °C and supplied chilled water of
17 °C, indoor air condition of 23 °C and 65 % relative humidity could be achieved during
daytime. In another field study, Sastry and Rumsey [25] conducted a side by side comparison
of radiant slab cooling and VAV all air system for the tropical climate of Hyderabad in India.
Radiant cooling design provided higher satisfaction among occupants although ceiling fan was
used in the indoor office space to raise the air movement level.
Low air movement in indoor space was the only identified comfort issue in the literature
for application of radiant cooling systems in the tropics. There is no minimum air velocity
requirement in the International standards like ASHRAE standard 55 [39] and ISO 7730 [40].
However, several local standards in tropical countries put some limit on the minimum
acceptable air velocity. Local Singapore standard of SS 554 [41] recommended indoor air
velocity to be in the range of 0.1 to 0.3 m/s. Malaysian local standard of MS 1525 [42] also
have the recommended range of 0.15 to 0.5 m/s for air movement level in the occupied space.
Yau and Hasbi [23] assessed thermal comfort and indoor air quality level in two green
buildings in Malaysia in which radiant slab cooling were implemented. The objective
measurements in the buildings revealed that indoor air condition provided by radiant slab
cooling is compatible with the criteria of local and International standards. The only identified
deficiency for this concept was low air movement which was below acceptable range in local
standards. In another study, Kwong et al. [43] conducted a thermal comfort survey in a green
building in Malaysia with slab radiant cooling and found air velocity below the limit set by
local guidelines.
2.3 System design/operation strategy
As explained in section 2.2, there are two main strategies in operating high temperature
cooling systems in the tropics. In the first strategy, supply chilled water temperature to HTC is
kept above outdoor dew point level and in the second strategy chilled water temperature is
kept below outdoor dew point and dehumidification is required to avoid condensation. The
main advantages of first scheme are utilizing passive chilled water system like cooling tower
and less risk of condensation and system complexity. This design is mostly suitable for night
time applications of residential buildings and personal fan has been advised to be used in
parallel for day time applications in commercial buildings. The second strategy is able to
provide adequate and accurate comfort condition under any indoor/outdoor scenarios.
However, parallel operation of radiant cooling and air based dehumidification systems makes
the design and control of overall system more complicated.
Radiant cooling panel (RCP), radiant slab cooling (RSC), active chilled beam (ACB) and
passive chilled beam (PCB) are the common types of HTC strategies. Each system has its own
advantages and drawbacks which makes it more suitable for some applications. RCP is a
temperature controlled surface which is attached to a chilled water hydronic system and
provides cooling into space mainly through radiation rather than convection. The infrared
image of a radiant ceiling panel in a conditioned lab is shown in Fig. 4. It can be seen that
temperature varies over the surface and it is colder in the areas which is in contact with water
pipes. Based on ASHRAE guideline [44], by assuming indoor air and average interior surfaces
temperature of 27 °C, capacity of radiant panel ranges between 15 to 115 W/m2 for water
supply temperature of 15 to 25 °C. For a combined radiant panel and natural air system in the
tropical climate of Thailand, radiant panel capacity was found to be in the range of 30-40
W/m2 with water supply temperature of 24-25 °C [16,45]. In another study in the tropical
climate of Singapore, Saber et al. [20] concluded that from morning till afternoon under
various supply water temperatures, radiant ceiling panel heat flux varies between 9 to 43
W/m2.
Fig. 4 Infrared image of a radiant cooling panel hung on ceiling [20]
In radiant slab cooling (RSC), water pipes are embedded into structure of building and it
provides mostly radiant cooling from floor or ceiling. This system requires integration with
building structure and needs to be planned at design stage before concrete pour (Fig. 5). RSC
also employs thermal mass of structure in order to shift and reduce peak cooling load of
building. Compared to radiant cooling panel, this design is more suitable for spaces where
steady cooling is required for a length of time. RCP can adjust faster to change in cooling load
and have better acoustical dampening as well as the option to be integrated with other building
systems like lighting. International standard organization on building environmental design
section provided guidelines for design, dimensioning, installation and control of embedded
radiant cooling system into structure of building as thermally activated building systems
(TABS) [46].
Fig. 5 Water pipes in slab cooling system [25]
High temperature cooling system can be implemented in buildings through convective
based system of chilled beam in which chilled water passes through a heat exchanger located
inside the conditioned space. Passive chilled beam is usually suspended on the ceiling and
provides sensible cooling through convective heat transfer with surrounding air. Warm air
adjacent to coils gets cooler and denser so it moves downward and causes a natural convection
flow in the indoor space. The perforated metal casing in Fig. 6 is for providing comfortable
cooling with aesthetic advantages and typically it has 50 % open area. The implication of
passive chilled beam has not been explicitly explored in the literature for the tropical context
while it is known that PCB can provide fast response cooling and is suitable for spaces with
high cooling load requirements [25].
Fig. 6 A passive chilled beam in operation [47]
In high temperature cooling systems like RCP, RSC and PCB, water based and air based
systems are operating independently in parallel. In active chilled beam design, conditioned air
passes through heat exchanger to enhance heat transfer mechanism between air and pipes. The
schematic of an operating active chilled beam is shown in Fig. 7. The primary ventilation air
mixes with secondary room air to supply dehumidified and cool air into space. In the tropical
context, the main design consideration for implementation of ACB is to avoid condensation on
beam and maintain dry cooling in the units. Kosonen and Tan [15] investigated an
implemented case of ACB in Singapore office building and concluded that condensation in
ACB unit is possible to be avoided by minimizing infiltration, supplying sufficient
conditioned air and adequate control of system.
Fig. 7 An active chilled beam in operation [48]
An air based system is required to work in parallel to high temperature cooling device to
handle latent load and ventilation requirement of indoor space. This air system could be a
central DOAS-VAV, RAS-VAV design or even a decentralized DOAS or FCU located near
the conditioned space. In the ideal operational scenario, air system design accommodates
utilizing the most achievable cooling capacity from high temperature cooling system and it
satisfies main part of space sensible load. In DOAS design, supply air is only limited to
ventilation outdoor air and it can bring the opportunity to downsize AHU and reduce duct size
and fan energy use compared to the all air system. Khan et al. [28] conducted energy
simulation analysis for applications of radiant cooling system in the tropics and showed that
radiant cooling combined with DOAS can provide 12.8 % higher energy saving compared to
RSC-FCU design. The coupled design of decentralized DOAS-RCP [22,49,50] can bring
chilled water closer to the indoor space by using small decentralized air supply units integrated
into building structure and further cut the fan energy use. DOAS design suits spaces with air
tight façades and low latent load where dehumidified fresh air could bring down indoor dew
point below HTC operational temperature. For spaces with high infiltration rate,
reconditioning of return air (RA) in RAS design can result in a more efficient system rather
than DOAS. If latent load of space including infiltration of humid outdoor air and human load
could be handled by low volume supply air of DOAS, there would be no need to recondition
indoor air.
The air distribution type in high temperature cooling applications also varies depending on
types of HTC device and space cooling load characteristics. This could vary from mixing
strategy like ceiling supply – ceiling exhaust to displacement oriented distribution types like
DV or floor supply – ceiling exhaust. In the reported studies of HTC applications, air supply
temperature ranged between 14 to 19 °C and ventilation rate was around 1 to 2 lit/s/m2 or 5 to
10 lit/s/person based on 5 m2 per person occupancy rate. This level of ventilation is above the
minimum ventilation requirement of Singapore standard SS 553 [51] which is 5.5 lit/s/person.
The chosen range of supply temperature in this concept may not satisfy the local comfort
criteria for distribution strategy of DV and FS-CE. A numerical study on the implementation
of DDOAS-RCS under FS-CE distribution [52] concluded that with off coil air temperature of
14 °C, in the areas close to floor diffusers, air could be around 17 °C which raises the concerns
of cold feet for occupants seated there.
Dehumidification of ventilation air requires low temperature chilled water while HTC
system is designed to operate with high temperature chilled water. The selection of design and
control strategy for providing both LTCW and HTCW at the same time has an important
impact on optimal operation and energy efficiency of the system. The typical strategy is to
produce low temperature chilled water (6 °C) in central plants and employ three way control
valves to raise the supply temperature for HTC. The schematic of water hydraulic lines and air
ducts in a DOAS combined with radiant panel is shown in Fig. 8. The valve modulator (V2)
regulates the flow volume in mix of supply and return chilled water in pipes based on
feedback from temperature sensors (T3 or T4). Sastry and Rumsey [25] explored the efficiency
and robustness of different control strategies and found that controlled valves based on fixed
return water temperature perform the best. The second strategy is to keep chilled water loop of
HTC and LTC separate and use a water to water heat exchanger to transfer the cooling to HTC
loop. The schematic of a VAV-RCP with two separate hydronic system loops is shown in Fig.
9. The return water from AHU passes through a heat exchanger to provide a supply chilled
water of 15 °C for radiant ceiling panel operation. This strategy has been implemented mostly
in research labs to have a better control on operation of high temperature cooling device
[27,45]. The third strategy is to use two separate chillers and water hydronic loops for HTC
and LTC systems as explained in Section 2.1 (Fig. 3). This scheme has the potential to
introduce further energy saving opportunities especially for the applications where more than
one chiller has been designated for cooling of building.
Fig. 8 Water hydronic lines and air streams in a DOAS combined with radiant panel [53]
(SF: supply fan, RF: return fan, P 1-2: pumps, C/C: cooling coil, T 1-11: temperature sensors, D 1-2: Dampers,
V 1-2: valve modulators, EW: Enthalpy wheel, RP: radiant panel, CH 1-2: chillers)
Fig. 9 Employing a heat exchanger to provide high temperature chilled water in a VAV-
RCP system [27]
Because of concerns regarding to condensation in tropical buildings, the start up and shut
down strategy of HTC system needs to be well planned and followed. Kosonen and Tan [15]
concluded that air based system needs to be switched on 30 minutes before water based
system in the morning in order to get rid of stored humidity in the space over the night. This
amount of time was estimated for a building with air tight façade and it could vary based on
characteristics of building. It is also advisable to switch off HTC device before air based
system to avoid any possible condensation after scheduled occupancy. In the case of slab
cooling system implementation, energy saving concerns also comes into play in choosing the
right start up and shut down strategy. Because of stored cooling capacity in slab due to thermal
mass, it is required to operate the radiant system in a shifted schedule compared to occupancy
schedule. One case study in the tropics recommended to switch on slab cooling system one to
two hours before occupancy and to switch it off one to two hours before unoccupancy [25].
Khan et al. [28] also explored various control strategies for operation of slab cooling system in
the tropical climate of Hyderabad in India and concluded that maximum energy saving can be
achieved when slab cooling system shut off four hours before end of occupancy schedule.
2.4 Investment and maintenance costs
Investment cost required for different HVAC systems is usually the key decision factor for
selection of a design especially when building investors and users are two different parties.
The cost of equipments and labors are geographic dependent and they also could vary based
on market demand and scale of manufacturing. A study conducted side by side comparison of
VAV and slab cooling system in the Hyderabad of India and concluded that capital cost of
DOAS-RSC is slightly lower than VAV system [25]. The cost saving opportunities from
reduction of AHU and duct size was about 6 % of total cost of ACMV system while radiant
piping, accessories and installation raised the investment cost for around 23 %. The study
considered 33 % reduction in HVAC low side work or commissioning cost in the case of
radiant cooling. In another study, Mumma [4] compared the first cost of DOAS-Radiant panel
with all air VAV system for a six storey building in the context of Philadelphia, in US. He
showed that total cost benefits of radiant cooling system through reduction of AHU, ductwork
and floor to floor height as well as integration with electrical and fire suppression services
could outweigh the added cost of radiant panels. However, for the application of active chilled
beams in the context of California in US, Stein and Taylor [54] estimated that total cost of
DOAS-ACB system is about 2.5 times of VAVR design. They considered additional labor and
subcontractors costs for ACB design and installation.
High temperature cooling concept is a new design for the tropical context and its practical
implications have not been investigated explicitly for this climate. Having cooling coil and
chilled water inside the conditioned space requires further inspection and possibly higher
maintenance cost. In the case of active and passive chilled beam, a regular cooling coil
cleaning service is required to avoid losing part of units’ capacity over time because of
collected dust on coils. The panel metal sheet and chilled water pipe behind the radiant cooling
panel also may require regular cleaning service. In order to reduce the risk of condensation in
applications of slab cooling system in the tropics, it was advised to insulate all water pipe
connections and manifolds [25].
3. Integration with energy recovery systems
In applications of high temperature cooling system, DOAS as the parallel air based system
can provide better performance and lower first cost opportunities compared to RAS due to
reduction in the size of AHU, ducting and fan energy use. The enthalpy difference between the
two air streams of fresh outdoor and indoor exhaust is significant and an energy recovery
system can reduce part of ventilation cooling load. Different types of ERS devices have been
introduced and investigated in the literature and industry. Rotary desiccant dehumidifier
(RDD) is one of these designs which incorporates a rotary wheel with desiccant coated
honeycombs for exchange of humidity. In active type of rotary desiccants (Fig. 10), a heat
source is required to activate the regeneration air streams in order to enhance the mass transfer
mechanism in the wheel. The warm and humid process air gets dehumidified after exchange of
humidity with regeneration air and then it is supplied into indoor space. The required heat
source is the main constraints for this design which is not easily accessible in the tropical
climate. Alongside this design, there is passive type of rotary desiccant in which the wheel is
placed between OA and EA streams and energy recovery happens without help of heat source.
In the passive RDD applications, the wheel needs to rotate at much faster speed compared to
active types [55].
Fig. 10 Active type of rotary desiccant wheel with air heater [55]
Several studies in the literature tried to explore the application of high temperature cooling
system combined with active rotary desiccant dehumidification in the tropics. Ameen and
Khizir [14] evaluated the performance of an active desiccant dehumidification system coupled
with chilled ceiling in the tropical climate of Malaysia. They found that this combination has
superior performance over conventional system in applications where high ventilation rate is
required like in theatres and hospitals. In another study in Singapore, Wahed et al. [18]
conducted energy simulation to investigate the performance of a thermally regenerated
desiccant system integrated with chilled beam in the warm and humid climates. They
modelled the active RDD with three different heating sources for regeneration process
including electric heater, solar collector and waste heat. The annual energy simulation and
economic analysis revealed that solar assisted dehumidifier integrated with active chilled
beam is the most cost effective design which consumes 10-20 % less energy than conventional
VAV system in the tropics.
Membrane based air to air heat exchanger is another type of energy recovery systems
which is getting wide range applications in the industry. In this design, heat and humidity
transfers between two air streams happen at a porous membrane surface without any active
parts inside the device. Air streams inside the heat exchanger can interact in different
arrangements as parallel, counterflow or cross flow streams [56]. During this interaction,
outside air loses some level of its heat and humidity to dry and cool exhaust air leaving the
indoor space. Nasif et al. [57] evaluated the performance of a Z type parallel plate heat
exchanger (Fig. 11) made of porous membrane. In this Z type design configuration, three
arrangement scenarios of parallel flow, counter flow and cross flow happen between two air
streams inside the heat exchanger. The results of experiments and annual energy simulations
showed that implementing this enthalpy heat exchanger in air conditioning system can result
in 5.7 to 9 % energy saving for the tropical climate of Kuala Lumpur.
Fig. 11 “Z” type membrane based heat exchanger [57]
4. Direct connection to cooling tower
Incorporating high temperature cooling systems in building could bring the opportunity to
implement passive chilled water systems like cooling tower to provide the required HTCW
without use of refrigeration system. Outdoor wet bulb temperature is the key parameter for
this design which usually ranges between 25 to 26 °C in the tropics except in the dry season
for the tropical savanna climate. Cooling tower can provide chilled water in the range of 25 to
26 °C for the radiant cooling system. However, this level of chilled water temperature cannot
provide effective cooling in HTC device for day time applications in office spaces. Vangtook
and chirarartaran [17] investigated different operational and design scenarios for the
application of cooling tower to provide high temperature chilled water for radiant cooling
panel in Thailand. The results showed that this system can provide adequate indoor condition
for spaces with low heat gain in night time applications. Tantiwichien and Taweekun [24] also
investigated the performance of radiant cooling panel using chilled water from an employed
cooling tower in the tropical climate of Thailand through experiments and simulations. The
results showed that this design is able to provide comfortable indoor condition during night
time and early morning with 41 % energy saving compared to split-type air conditioner. The
performance of incorporated cooling tower in this design was around 60 % which varied
depending on outdoor dry bulb and wet bulb temperatures.
5. Discussions
The whole idea behind implementing high temperature cooling systems in buildings is to
introduce a more energy efficient cooling system compared to the conventional all air system.
A precise and detailed design strategy needs to be followed in order to truly utilize the
potential benefits of HTC systems. It was shown by several studies in the literature [17,28]
that fan energy use can be reduced by 50 to 70 % through incorporating DOAS combined with
radiant cooling system. Downsizing fan and duct size in ACMV system compared to
conventional design is the key factor for achieving higher energy saving in this concept.
Providing an air tight façade for the indoor space is necessary to minimize the infiltration rate
and latent load of space, especially in the tropical context where outdoor humidity is high all
year round. In the ideal operational scenario, the amount of supply air volume is in the range
of minimum ventilation requirement of space and HTC plays a significant role in satisfying
the sensible load of space.
HTC systems operate with high temperature chilled water which can bring the potential to
enhance the operational efficiency of chiller in buildings. Two separate chillers could to be
installed in buildings to provide LTCW and HTCW for air and water systems, respectively.
This design strategy suits the buildings where multiple chillers were assigned to be
operational. The potential energy saving of this separation can be understood from the reverse
theoretical thermodynamic cycle of Carnot. This cycle is the most efficient possible set of
processes to transfer a certain amount of heat from a cold source (QC-TC) to a hot one (QH- TH)
with a given amount of usable work (W). The COP of the cycle in the tropical context
(average outdoor temperature of 30 °C) for the low temperature (6 °C) and high temperature
(16 °C) cooling operations can be determined as follows,
Based on the reverse theoretical Carnot cycle, there is a 77 % (Eq. 2-3) increase in
operational COP of chillers for high temperature cooling applications in tropical buildings.
However, because of the exergy losses in different sub-processes inside the chiller, the COP of
actual refrigeration systems operating in cooling system of buildings are far below the
theoretical Carnot values. The level of increase in COP for high temperature condition
compared to low temperature cooling scenario is also less likely to be in the same order. The
chillers in building cooling system are usually designed and regulated to efficiently provide
chilled water at temperature of 6-7 °C which matches the requirement of conventional air
conditioning system in the buildings. Custom designed chillers for low temperature lift
conditions have been introduced and investigated by some research institutes and
manufacturing companies for high temperature cooling applications or other process cooling
requirements. Doyon [34] investigated the critical design parameters of centrifugal chillers for
superior efficiency at low temperature lift conditions. He found open-drive/gear-drive
compressors with variable orifice and oil education system as the most suitable features in
chillers for high temperature cooling applications. He also concluded that chillers with
hermetic drive motors would not be able to utilize the potential energy saving of HTC
operational conditions due to required high head pressure for cooling of motor windings. In
another investigation, Wyssen et al. [58] designed a chiller prototype for low temperature lift
conditions which was equipped with a semi-hermetic reciprocating compressor. The outcomes
of experiments showed that at evaporative temperature of 15 °C, the COP of this chiller
increases from 4 at 40 °C temperature lift to about 11.4 at 13 °C lift. The COP of this custom
designed chiller as well as ideal Carnot and typical chillers are plotted in Fig. 12. It can be
seen that performance curve of this prototype chiller fits in between typical chillers and ideal
efficiency curves. The low lift chiller performs better than air cooled/water cooled chillers
both in terms of COP value at any specific lift and increase rate for lower temperature lift.
This shows some part of exergy losses in chiller sub-processes has been eliminated in this
design.
Fig. 12 COP of ideal Carnot cycle, low lift chiller and two typical air cooled/water cooled
chillers at evaporative temperature of 15 °C
The application of high temperature cooling in tropical buildings can be categorized based
on their supply chilled water temperature to HTC device. When the supply water temperature
is above the outdoor dew point level and in the range of 25 to 26 °C, it can only provide
comfortable indoor space for night time applications. This design scheme could also be used
during day time in office buildings with low space heat gain for occupants under light clothing
level when personalized or ceiling fans are used to raise the air movement on work stations. In
the second design scheme, chilled water is supplied to HTC at temperatures lower than
outdoor dew point and comfortable indoor space can be achievable under any indoor/outdoor
scenarios. In this scheme, the parallel conditioned air system would keep the indoor dew point
below HTC operating temperature to avoid condensation. The only reported deficiency in the
literature for this conditioned indoor space is the low air movement as locally acclimatized
occupants in the tropics prefer to be exposed to higher air speed. This issue has been resolved
by using a ceiling fan in the office space for an implemented case of radiant cooling system
[25].
Different strategies have been investigated and suggested in the literature for the design
and operation of HTC in tropical buildings. A detailed control strategy is required to be
followed especially in the cases where HTC operational temperature is below outdoor dew
point to avoid condensation. The start up and shut down schemes could vary based on type of
HTC system and characteristics of buildings. In the case of slab cooling system, a shift needs
to be considered between operational schedule of HTC and occupancy period. For other types
of HTC systems located in the indoor space, the air based system should be switched on /off
before/after water based system in the morning and evening, respectively. The choice of air
conditioning and distribution system could also play an important role in energy saving and
thermal comfort of occupants. DOAS was found to be the best parallel air based system and
from energy efficiency point of view, the amount of supply air is best to be kept near
minimum ventilation requirements of space. In spaces with high latent load, supply air volume
to space may be required to exceed this minimum requirement. Since in this design scenario,
the off coil temperature would be in the range of 14 to 16 °C, it is advisable to implement
ceiling supply-ceiling exhaust distribution instead of displacement strategy to avoid cold feet
for occupants.
The initial cost of high temperature cooling system varies based on HTC device types and
radiant slab cooling is the least costly choice. However, this design needs to be planned at
early stages before construction of building structure. The added cost of piping and accessories
to overall ACMV design could be mitigated by reduction of fan and duct size for circulation of
air in buildings. Less floor to floor height and integration with lighting and fire suppression
systems are other potential opportunities for saving material and space per floor area. In order
to utilize these potential benefits for implementation of high temperature cooling system, the
coordination of building structural, mechanical, electrical and fire protection contractors is
required. The maintenance cost of HTC system is also expected to be higher due to
significantly higher water pipes compared to conventional all air system especially in the case
of passive/active chilled beams.
The advantages of water based system combined with DOAS can be further utilized if an
energy recovery system is incorporated into air handling unit. The high enthalpy difference
between outdoor and indoor air in the tropics makes implementation of ERS more
economically feasible. Several studies in the literature reported significant energy saving for
coupling of HTC and active rotary desiccant dehumidification in the tropics through
experimental setup or simulation. Solar assisted heating was found as the best strategy for the
required heating in the regeneration process, although this source is not reliable in the tropics
due to high annual median cloud cover. Membrane heat exchangers (MHX) have also been
explored in the literature in the tropical context and promising performance was observed for
this design [50].
A collection of high temperature cooling design strategies and components in tropical
building based on reported outcomes in the literature is shown in Fig. 13. User controlled
temperature and humidity sensors are considered in indoor space to provide necessary changes
on operation of water and air based systems based on different indoor/outdoor scenarios. With
increase of humidity or dew point temperature above the specified threshold, signal would be
sent to actuators on air supply units’ pumps and three way control valve to reduce water
supply temperature and/or increase water flow rate. This change would enhance
dehumidification capacity of air system to avoid any risk of condensation on radiant panel.
Three way control valve and pump on HTC hydronic system could modulate its capacity
based on feedback from temperature sensors to provide the required sensible cooling for
space. Installing CO2 sensors can also bring further value to this design for the thermal zones
where there is high variation of occupancy level like in meeting rooms. Based on these
sensors’ feedback, the volume of fresh air coming into space could be regulated through
modulating supply air fan speed.
In this design strategy (Fig. 13), an energy recovery system is designated for DOAS to
recuperate some part of cooling from exhaust air stream. This ERS device could be an
active/passive desiccant wheel or a membrane based air to air heat exchanger which transfers
heat and humidity between two air streams. The high enthalpy difference between indoor and
outdoor condition in the tropical context would make ERS an effective system for the
applications of DOAS. In addition, the required high temperature chilled water for HTC can
be provided through alternative strategies like cooling tower or a separate low lift chiller.
Direct connection of HTC to cooling tower without refrigeration system is mostly suitable for
spaces with low heat gain in night time applications. A custom designed chiller which operates
at considerably higher efficiency in low temperature lift applications can also be a more
efficient solution compared to a central low temperature chilled water (LTCW) system. The
level of improvement in COP of designated low lift chiller needs to outweigh the added cost of
having a separate chiller for HTC, in order for this alternative solution to be economically
competitive
Fig. 13 A selected design strategies and components for the application of high
temperature cooling in the tropics
6. Conclusions
The reported studies in the literature on the implementation of high temperature cooling
systems in the tropics were reviewed to get a better understanding on overall performance of
this cooling strategy for the warm and humid climates. Various aspects of this implementation
were explored in terms of energy saving and thermal comfort for locally acclimatized
occupants as well as optimal design and operational scenarios. The outcomes of this
investigation can be summarized in the following points,
HTC systems can bring the opportunity to downsize fan and duct size for air
circulation and cut the fan energy use by half compared to all air system
In the tropical context, an air tight façade is preferable for this strategy to
minimize the infiltration rate or in other words the latent load of space
Incorporating a custom designed chiller for low temperature lift conditions can
further utilize the potential energy saving of this concept
When supply chilled water temperature to HTC is above the outdoor dew point
level, comfortable indoor condition can be achieved only under specific
indoor/outdoor conditions. This high temperature water can be provided with
direct connection to cooling tower without use of refrigeration system in the
tropics.
When supply chilled water temperature to HTC is below the outdoor dew point
level, comfortable condition is achievable under any indoor/outdoor conditions.
Although a parallel conditioned air system is required to avoid condensation on
HTC device. The low air movement in this indoor space was the only reported
comfort concern for locally acclimatized occupants in the tropics.
A control strategy needs to be followed to avoid condensation in start up and shut
down period of system in tropical buildings. In the case of slab cooling system, a
shift between operational schedule of HTC and occupancy period needs to be
considered.
DOAS with ceiling supply-ceiling exhaust was found as the most suitable and
efficient air conditioning and distribution strategy for HTC implementation.
Among HTC cooling systems, slab cooling can be implemented at lower initial
cost. However, it requires early-stage planning and integration into structure of
building.
Incorporation of energy recovery system into DOAS can reduce ventilation
cooling load of space and improve the overall performance of HTC design.
Membrane based air to air heat exchangers showed promising performance in the
tropical context.
The practical implications of high temperature cooling system are required to be further
explored through actual implementation of this concept into tropical buildings. This coupled
air/water system demands a more sophisticated design and control strategy compared to all air
system. Thoughtful decisions need to be made at design stage based on experience of past
projects and building performance simulation for a successful implementation of high
temperature cooling strategy in the tropics.
7. Acknowledgment
This work was established at the Singapore-ETH Centre for Global Environmental
Sustainability (SEC), co-funded by the Singapore National Research Foundation (NRF) and
ETH Zurich in collaboration with department of building of NUS.
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